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© Raymond Pettibon 


RESEARCH LIBRARY 
emmy) RESCARCH INSTITUTE 


N MOORE ANDREAS COLOR CHEMISTRY LIBRARY FOUNDATION 


mri) | OGRAPHY 


ITS PRINCIPLES AND PRACTICE 


A Manual of the Theory and Practice of Photography 
Designed for Use in Colleges, Technical Institutions 
and by the Advanced Student of the Science. 


By 
eo, NEBLETTE, A.R.P.S. 


Member of the Faculty of the Texas A. and M. College 
Formerly Director, Division of Photography, The Pennsylvania State College 


NEW YORK 


D. VAN NOSTRAND COMPANY 
EIGHT WARREN STREET 


1927 


All rights reserved, including that of 
into the Scandinavian and other 


ane” 


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| 


Par 
ae 


Si, 


PREFACE 


Manifold as are the applications of photography in all branches of 
science and industry and great as has been the increase in our knowl- 
edge of its basic principles in recent years, comprehensive and ade- 
quate instruction in the subject has been largely neglected by our uni- 
versities and technical institutions. Despite its daily application to 
the practice of almost every branch of science and industry, and in- 
deed in every walk of life, as well as its importance from the stand- 
point of pure science, there is not, within the knowledge of the writer, 
a single university or technical institution in this country which offers 
a thorough and complete course in the science and practice of photog- 
raphy. 

The literature of photography is widely scattered among a large 
number of journals, some of which have long since disappeared, while 
until comparatively recently no worthy attempt had been made to- 
wards the abstracting and indexing of photographic information. 
Excluding papers in the periodical press, photographic literature can 
for the most part be divided into two classes: (1) works of an ele- 
mentary nature designed for the beginner and paying but scant at- 
tention to the fundamental scientific basis of the subject and (2) 
works of an encyclopedic nature designed principally for reference 
purposes, such as Dr. J. M. Eder’s monumental work in German, the 
Ausfiihrliches Handbuch der Photographie and Fabre’s Traite En- 
cyclopedique de Photographie in French. Valuable as these works 
may be, they are not textbooks in the true sense of the word and there 
is in fact no work dealing both with the science as well as the practice 
of photography which is especially adapted for use as a text. 

The present work is an attempt to meet that need. It embraces the 
features which several years’ experience in the teaching of the subject 
has shown the writer to be desirable in a work designed for college 
instruction. No attempt has been made to compile a complete treatise 
on the subject, while at the same time the fact has been kept in mind 
that a superficial treatment of the subject, one which is concerned with 
effects rather than causes and with operations rather than scientific 
principles, is undesirable in a work of collegiate grade. Accordingly 

iil 


iv PREFACE 


it has been the aim of the writer throughout to present as clearly and © 
as concisely as possible the fundamental principles of the science of 
photography, omitting nothing of primary importance necessary to an 
understanding of the subject and paying particular attention to the 
proper codrdination of the facts to one another. 

The practical side has not been lost sight of, however, and several 
of the chapters deal with their subject more from the standpoint of 
technique rather than science. These, it is hoped, will render the 
work useful not only to the student but to the practical worker as well. 

An apology, perhaps, is due for the omission of certain subjects and 
for the brief treatment accorded to others. This was, however, to a 
certain extent, demanded by the scope of the work which is that of a 
text rather than a treatise. Accordingly a large number of the un- 
settled controversies have either been omitted, or but briefly treated, 
as it was felt inadvisable to consider in a work of this nature subjects — 
which still await satisfactory solution. 

Footnotes throughout the text will show the extent to which | am — 
indebted to others, while to the following authorities I desire to place 
on record my appreciation of their invaluable services: Dr. C. E. K. | 
Mees, Dr. S. E. Sheppard, Dr. A. P. H. Trivelli, all of the Eastman 
Research Laboratory; Mr. F. F. Renwick; Dr. Walter Clark of the 
British Photographic Research Association ; Dr. Hermann Kellner of 
the Scientific Department, Bausch and Lomb Optical Company ; Carl 
J. Reich of the Gundlach-Manhattan Optical Company; Mr. George 
E. Brown, Editor of the British Journal of Photography; Mr. Frank © 
Roy Fraprie and Mr. E. J. Wall of American Photography; Drs. 
Walters and Davis of the Bureau of Standards; and Miss Bess 
Spence of this institution who has assisted me in seeing the work 
through the press. To all others who have assisted in the preparation 
of this work in any way, a cordial acknowledgment of appreciation is 
also due. 

C. B. NEBLETTE 
CoLLEGE STATION, TEXAS, 1926 


re ee eet ape en ee, 


CONTENTS 


CHAPTER I. The Development of Photography.............. 


Introduction. The Development of the Camera. Jean Baptiste 
Porta. The Camera Obscura with Lens. Early Records of the 
Photochemical Action of Light. The Forerunners—Davy and 
Wedgwood. Life and Work of Joseph Nicephore Niepce. Life 
and Work of Jacques Mande Daguerre. The Daguerreotype Proc- 
ess. Later History of the Daguerreotype. The Positive Process 
of Bayard. Life and Work of Henry Fox-Talbot. The Calotype 
Process. Miscellaneous Paper Processes. Introduction of Glass. 
Scott-Archer and the Introduction of Collodion. The Collodion 
Process. Inconveniences of the Collodion Process. Modifications 
of the Collodion Process. Introduction of Collodion Emulsion. In- 
troduction of Gelatino-Bromide Emulsion. Improvements in the 
Gelatino-Bromide Process. Development of Printing Processes with 
Silver Salts. Platinum Printing Processes. Printing Processes 
with Bichromated Colloids. 


eter ee ne Camera and Darkroom................... 


The Box Camera. The Miniature Camera. Folding Hand Cam- 
eras. The Professional Camera. The Reflex Camera. The Prin- 
cipal Adjustments of Cameras. The Swing Back. The Reversible 
Back. Other Movements. The Darkroom. Ventilation. Size. Ar- 
rangement. Water Supply. The Illumination of the Darkroom. 
The Safelight. The Efficiency of Darkroom Safelights. Trays, 
Tanks and Graduates. Miscellaneous Features. 


eet notoriapiic Optics.............. 02.0 eee 


Introduction. Refraction of Light. Dispersion. Lenses and Image 
Formation. Image Formation according to the Gauss Theory. The 
Position of the Nodes. The Principal Focus of a Lens—Focal 
Length. Focal Length and Size of Image. Angle of View. Con- 
jugate Focal Distances. Extra Focal Distances. Theory of Depth 
of Focus. Factors Controlling Depth of Focus. The Intensity of 
the Image. Speed of Lenses—Systems of Diaphragm Notation. 
Effective Aperture. Loss of Light in Lenses Due to Absorption 
and Reflection. Variation in Relative Aperture with Distance of 
Subject. 


CuHapTER IV. The Aberrations of the Photographic Objective. . 


Introduction. Chromatic Aberration. Spherical Aberration. Coma. 
Curvature of Field. Distortion. Unequal Illumination. Astig- 
matism. Flare and Flare Spot. 


Vv 


vil 


CONTENTS 


CHAPTER V. The Photographic Objective. .... 7.90 


PartI. THE AsticMats: The Single Collecting Lens. The Single 
Achromat. Semi-Achromatic or Soft-Focus Lenses. The Aplanat 
or Rapid Rectilinear. The Petzval Portrait Lens. 

Part II. THe ANaAsticMaTs: Introduction. Cemented Symmetri- 
cal Anastigmats. Alternate Form of the Double Anastigmat. The 
Four Glass Element—the Protars. The Five Glass Element. Sym- 
metrical Lenses with Air-Spaces. The Gauss Construction. Stein- 


heil’s Unofocal. Graf Variable and Anastigmat. Beck’s Neostig- — 


mat and Isostigmat. The Plasmat. Dallmeyer’s Stigmatic. Ru- 
dolph’s Early Protars. The Unar. The Tessar. Combination of 
Air Space and Cemented Surface—Later Developments. Serrac. 
X-Press. Radiar. The Cooke Triplet. Development of the Cooke 
Triplet after H. D. Taylor. The Aviar. Aldis. Heliar, Dynar. 
Pentac. Ernostar. 

Part IIJ. THe Terropyective: Principle. The Compound Tele- 
objective. Early Fixed-Magnification Teleobjectives. Anastigmatic 
Fixed-Magnification Teleobjectives. Dallmeyer’s Adon. 


CHAPTER VI, The Photographic Emulsioniyag2 5 a oe 


CHapTrer Vil. Orthochromatics: ...... 3. he ae 


Introduction. The Two Classes of Emulsions. General Outline 
of Operations in Emulsion Preparation. Gelatine. Light Sensitive- 
ness of Silver Salts. The Preparation of Emulsions. Emulsifica- 
tion. Gelatino-Bromo-Iodide Emulsions. Digestion of the Emul- 
sion. Fog. Theory of Digestion. Eliminating the Soluble Salts. 
The Silver Bromide Grain of Photographic Emulsions. The Sensi- 
tivity of the Silver Halide Grain. The Nature of the Sensitivity 
Substance. Grain Size and Distribution and Its Relation to the 
Photographic Properties of Emulsions. 


Light and Color—the Spectrum. Visual and Photo-Chemical Lumi- 
nosity. History of Dye Sensitizing. Known Facts Regarding Color 
Sensitizing. Color Sensitizing by Bathing. Sensitizing for Green 
and Yellow. Sensitizing for Red. Mixtures of Dyes as Color 
Sensitizers. The Theory of Light Filters. Orthochromatic Filters. 
Contrast Filters. Orthochromatic Methods in Landscape Photog- 
raphy. Orthochromatic Methods in Portrait Work. Photograph- 
ing Color Contrasts. 


CuaApTER VIII. The Latent Photographic Image............. 


Photo-Physical and Photo-Chemical Change. The Latent Image. 
Artificial Latent Images. Hydrogen Peroxide. Sodium Arsenite. 
Reversal by Chemical Reagents vs. Reversal by Light. Photo-Re- 
gression. The Action of Solvents of Silver on the Latent Image. 
Physical Development of the Latent Image after Fixation. The 


CONTENTS vil 


Photosalts. Image Transference. Indoxyl Development. Action 
of Oxidizing and Halogenizing Agents on the Latent Image. Re- 
versal by Light. Theories of the Latent Image. The Oxy-Halide 
Theory. The Sub-Halide Theory. Evidences for the Liberation of 
Halogen. Do Silver Sub-Halides Exist? Objections to the Sub- 
Halide Theory. The Metallic Silver Theory. The Molecular Strain 
Theory. Evidence for the Molecular Strain Theory. The Electron 
Theory of the Latent Image. The Photo-Electric Effect. Evidence 
for and against the Electron Theory. The Colloidal Silver Theory. 
Sheppard’s Orientation Hypothesis of the Latent Image. 


ESTES G0 226. 


What is Sensitometry? Resumé of Sensitometric Investigation. 
Instruments for Sensitometric Investigation. Standard Light 
Sources. Sensitometers. Instruments for the Measurement of Den- 
sities. Opacity—Transparency—Density. Exposure and Develop- 
ment of Sensitive Materials for Speed Determination. Relation of 
Exposure and Growth of Density. The Characteristic Curve. The 
Significance of the Characteristic Curve. Inertia as an Inverse 
Measure of Speed. Variation of the Inertia. Watkins Central 
Speed Method. Wedge Methods of Sensitometry. The Perfect 
Negative. Density-Exposure Relation and Correct Reproduction. 
Latitude of Sensitive Materials. Development and the Reproduction 
of Contrasts. Constant Density Ratios. An Important Difference. 
Development and Contrast. Gamma as a Measure of Contrast. 
Gamma and the Characteristic Curve. Calculation of Gamma. 
Gamma Infinity. 


CHAPTER X. The Exposure of the Sensitive Material......... 254 


The Problem. Light Intensity and Exposure. The Subject. Speed 
of Plate. Speed of Lens. Determination of the Time of Exposure. 
Exposure Meters. Corrections for Special Subjects. Visual Meters, 
Types, Principles and Use. 


ee eee ne: Theory of Development. i .:..5.0... 6.5 266 


Introduction. The Invasion Phase. The Chemical Reaction within 
the Cell—The Reduction Phase. The Precipitation Phase. De- 
velopment as a Reversible Reaction. The Action of Sulphites, Solu- 
ble Bromides and Alkali in Organic Developing Solutions. The 
Physical Chemistry of the Developing Process. The Induction 
Period. The Velocity of Development. The Velocity Constant. 
Calculation of the Time of Development for a Given Gamma. Ef- 
fect of Temperature on Development. Calculating the Temperature 
Coefficient. Time of Development at Various Temperatures. The 
Action of Soluble Bromides in Development. The Relative Reduc- 
ing Energy of Developing Agents. 


Vill 


CONTENTS 


CuHapter XII. Organic Developing Agents................- 


Developing Power. Classification of Developing Agents. The 
Source of Organic Developing Agents. The Significance of Group 
Relations. Slow and Rapid Developers. In Explanation. Adurol. 
Amidol. Preservatives of Amidol Solutions. Certinal. Edinol. 
Eikonigen. Glycin. Hydrochinon. Metol. Metoquinone. Mono- 
met. Neol. Ortol. Paramidophenol. Paramidophenol-Phenolate 


Compounds. Pyrocatechin. Pyrogallol. Minor Organic Develop- . 


ing Agents. 


CuapTeR XIII. The Technique of Development............. 


Introduction. The Sulphites in Development. The Alkalis in De- 
velopment. The Value of Desensitizers. The Development of De- 
sensitizing Agents. Desensitizing in Practice. Development by In- 
spection. The Watkins System of Factorial Development. What 
Determines the Factor. Accuracy of the Factorial System. Thermo 
Development. The Watkins System of Thermo Development. De- 
veloping Speeds of Commercial Plates. Developers. Instructions. 
Thermo-Development with Colycin. The Efficiency of Time De- 
velopment. ~ 


CHAPTER XIV. The Laws of Fixation and Washing.......... 


Action of Sodium Thiosulphate on the Silver Halides. The Mecha- 
nism of Fixing. Influence of the Concentration of Thiosulphate and 
Temperature on Time of Fixation. Influence of Ammonium Chlo- 
ride on the Rapidity of Fixation. When are Plates Fixed? Ex- 
haustion of the Fixing Bath. The Fixation of Prints. Plain Fixing 
Baths. Acid Fixing Baths. Acid Fixing and Hardening Baths. 
Troubles with the Acid Fixing and Hardening Bath. Extra Hard- 
ening Baths. The Mechanism of Washing. The Efficiency of 
Washing Devices. The Washing of Prints. Methods of Deter- 
mining the Presence of Thiosulphate. Hypo Eliminators. 


CuarteR XV. Defects in Negatives......... 33) 


The Why of Defects. Thin Negatives. Dense Negatives. Fog on 
Negatives. Local Fog. General Fog due to Light. Chemical Fog. 
Dichloric Fog. Developer Stains. Silver Stains. Miscellaneous 
Stains. Transparent Spots. Opaque or Semi-Opaque Spots. Mis- 
cellaneous Troubles. 


CHapTerR XVI. _ Intensification and Reduction.,.. ).9 eee 


Part I. Repuction. The Three Classes of Reducers. Farmer’s 
Reducer. Mercury and Cyanide. Iodine Cyanide. Belitiski’s. Per- 
manganate. Bichromate. Proportional Reducers. Super-propor- 
tional Reducers. Theories of Super-proportional Action. Practice 


CONTENTS 


of Persulphate Reduction. Intensification, Definition, Methods and 
Characteristics. Mercury Intensifiers. Monkhoven’s. Mercuric 
Iodide. Silver Intensifiers. Chromium Intensifiers. Uranium. 
Sulphide. Lead. Copper. Sensitometry of Intensification. Local 
Reduction and Intensification. 


CHAPTER XVII. Printing Processes with Silver Salts......... 


Part I. DeEvELopING Papers: Characteristics of Development 
Papers. Adapting the Paper to the Negative. Exposure. De- 
velopers. The Safelight. Development. Factorial Development. 
The Proper Factor. The Short Stop. Fixing. Washing. Drying. 
Alteration of .Contrast. Reduction and Intensification of Prints. 
The Glazing of Prints. 

Part II. GetatinE P-O-P: Toning. Instantaneous Toning. 
Black Tones with P-O-P. Fixing. 


pee ioe Projection Printing... 0.2.2.2 ee eee 


Introduction. Fixed Focus Enlarging Cameras. Apparatus for 
Projection Printing with Daylight. Apparatus for Projection Print- 
ing with Artificial Light. Self-focusing Apparatus for Projection 
Printing. Illuminants for Projection Printing. The Mercury- 
Vapor Lamp. The Electric Arc. Incandescent Lamps. Securing 
Even Illumination without Condensers. The Condenser in Projec- 
tion. Condensing Lenses with Diffusing Media. The Projection 
Lens. The Projection Easel. The Negative for Projection Print- 
ing. The Technique of Projection Printing. Focusing. Determin- 
ing Exposures in Projection Printing. Relative Exposure, Scale 
and Aperture in Enlarging or Reduction. Introducing Clouds in En- 
largements. Enlarged Negatives. Sensitive Materials. _ Exposure. 
Development. 


Serer ee | ne foaritern Slide... 0.5... ea. 


The Lantern Slide and Its Uses. The Negative. Lantern Plates. 
Printing Frame for Contact Printing. Exposing. Printing by 
Projection. Developers. Development. Fixing, Washing and Dry- 
ing. Masking. Spotting. Binding. Advertising Slides. Toning 
of Lantern Slides by Restrained Development. Physical Develop- 
ment. Colors on Development with Thiocarbamide. Toning of 
Lantern Slides. Reduction and Intensification of Slides. 


CHAPTER XX. The Toning of Developed Silver Images....... 


Introduction. The Sulphur Toning Processes—the Print. The 
Hypo-alum Process. Zanoff’s Controlled Hypo-alum Method. Sul- 
phur Toning with Acid Hypo. Toning with the Polysulphides. 
Single Solution Sulphur Toning Processes (Shaw’s Process). The 


1X 


x CONTENTS 


Indirect Process of Sulphide Toning. Rebleaching of Sulphide 
Toned Prints. Indirect Sulphide Toning with Intermediate Develop- 
ment. Mercury Sulphide Toning (Bennett’s Process). Toning with 
Copper. Toning with Uranium. Iron Toning Processes. Toning 
with Vanadium. Minor Toning Processes. 


CHAPTER XXI._ Platinotype and Iron Printing Processes...... 479 


Introduction. Theory of the Process. Commercial Papers and their 
Treatment. Exposure. Development. Variations in Contrast. 
Variations in Color. Silver Platinum Papers. The Kallitype Proc- 
ess. Blue Printing. . 


CHAPTER XXII. Printing Processes Employing Bichromated 
Colloids. I. (Carbon and Carbro).... 489 


Part I. Historica: Sensitiveness of Chromic Compounds and 
Bichromated Colloids. The Development of the Carbon and Gum- 
Bichromate Processes. The Development of the Oil, Bromoil and 
Powder Processes. The Chemistry of Pigment Printing with 
Bichromated Colloids. 

Part II. THe Carson AND CARBRO ProcESSES: Introduction. Car- 
bon Tissues. Double and Single Transfer. Sensitizing the Tissue. 
Exposure. Development. Double Transfer. . Transferring to 
Rough Papers. The Carbro Process. The Bromide Print. Sensi- 
tizing the Tissue. Transfer. Redevelopment of the Bromide Print. 
Development of the Carbro. Carbon on Bromide. Multiple Print- 


ing. 


CHAPTER XXIII. Printing Processes Employing Bichromated 
Colloids. II. (Gum-Bichromate and 
- Allied Processes) . 2. oer 3 eon 511 


Introduction. Materials. The Negative. Formulas. Effect of 
Varying Proportions of the Coating Mixture. Coating. Drying. 
Exposure. Development. Registration. Gum-Bromide and Gum- 
Platinum. The Powder Processes. Formula of E. J. Wall. Res- 
inopigmentype. 


CHAPTER XXIV. Printing Processes Employing Bichromated 
Colloids. III. (Oil, Bromoil and Trans- 


Introduction. Materials for the Oil Process. Papers for the Oil 
Process. Brushes. Pigments. Sensitizing. Exposing. Pigment- 
ing. Incorrect Exposure. Drying and Mounting. Duvivier’s Proc- 
ess. The Bromoil Process. The Choice of the Paper for the Bro- 
mide Print. The Production of the Bromide Print. Bleaching of 
the Bromide Print. Chemical Theory of the Bleaching Operation. 
Fixing. Producing the Relief. Pigmenting. Namias Method of 


CONTENTS 


Pigmenting. Defatting the Finished Bromoil. Bromoil Transfer. 
The Bromide Print. Preparation of the Bromoil. The Transfer 
Paper. The Transfer Press. Transferring the Pigment. Zaeper- 
nick’s Chemical Transfer Method. Multiple Transfer. 


Peewee COPYING)... ee ee Ee eee sits 


Introduction. Apparatus for Copying. Methods of Illuminating 
the Print. Copying Cameras. The Objective for Copying. Focus- 
ing. Copying to Scale. Exposures in Copying. The Copying of 
Subjects in Pure Black and White. Development of Process Plates. 
Copying Photographs or Like Subjects in Monochrome. The Pho- 
tography of Colored Objects. Photography of Small Objects ‘in 
the Studio. 


CHAPTER XXVI. Natural Color Photography............... 


Introduction. Processes of Direct Color Photography—The Bleach 
Out Process. Processes of Direct Color Photography—Processes 
- of Light Interference. Natural Color Photography by Trichromatic 
Methods. Making the Three Color-Sensation Negatives. Additive 
and Subtractive Three-Color Photography. Subtractive Printing 
Processes. Multi-Color Screen Plates. The Autochrome Plate. 
The Compensating Filter. Handling of the Autochrome Plate. 
Exposure. Development. Reversal of the Image, Varnishing. 
After-Treatment of Autochromes. The Agfa Color Plate. Dupli- 
cating Processes of Screen-Plate Color Photography. The Duplex 
Method. 


Appenpix. A List of the More Important Reference Works on 


EE No ae tee sip ob Piss + ie 


eee Sena 10) LECHNICAL JOURNALS .........6.-.cecceeeee 


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ILLUSTRATIONS 


Fic. 
1. The camera obscura from an old print. (Courtesy of the Smithsonian 
RRO rr EO i Fk Api wie Va able hs wadsswldseeusecss 2 
2. Johann Heinrich Schulze. (From Eder’s biography)................. 6 
3. Thomas Wedgwood. (Chalk drawing, author unknown).............. 7 
4. Joseph Nicephore Niepce (Courtesy of the Société Francaise de Photo- 
Re eo pt | xc wPinw sw vlalk ¢ sflvibiale dle ala e's wile w © 9 
ME EN PU O TCO, ., ) . edie se ck es ec dje donde cede ee ueeucveues 10 
i. yA 'y G tate wale ples bis ca a diseesllewneweeeuces 10 
Beeenoeranuicuprims by Niepce, 1824. .... 0.6. nce cc ce ce nace enue II 
ES SE CS EB: fat oy a 12 
7. Tomb of Daguerre. (Courtesy of the Société Francaise de Photo- 
Ss rcs ios wider ule dia Lbs x oF wm dceelS a kp et ee Wye 14 
8. Fuming cabinet for the Daguerreotype process.................000008 15 
9. Developing box for the Daguerreotype process..............0..0 00008 16 
10. An early Daguerreotype portrait. Often stated to be the first portrait 
EMM Pe ea. Uk sca tle sb Sitia e vis webs Hele wertwemes 7 
Bam reer ietiry Ox 1 dIDOt. oo ni is ec ie hae etna eee ane 19 
12. Frederick Scott Archer. Drawing from an old print reproduced in J. 
Werge—The Evolution of Photography.............2.00 cece eens 22 
13. The wet collodion process in the field. (From an old manual)........ 24 
14. Portable laboratory for field use with wet collodion................... 25 
peemeeemtenped each Maddox. ...... 25556250 bee nse leew ececeaaewensunrs 25 
16. Typical miniature camera for plates and for roll film................ 36 
fy stan cameras tor plates and for roll film..........0-.0....0000.e00 38 
18. Professional view camera..................--0200- eM ath Gc, Saude 39 
Se evOr tiie Tenee Caticia... 2... 26. cette een eee deeeraes 41 
20. Principle of the swing back. Use of the swing back for securing _ 
TIME LOCUS. oo c sw inp a dian o's Sin cic etek mes cee ase nbn cas 43 
21. Ventilation of the darkroom. (Courtesy of Eastman Kodak Company) 47 
22. Floor plan of darkroom for amateur use......................-20 ee 48 
Sameer er airece Garkroom lamp............- 000-6 s cece eens ne ceeens 50 
24. Design for indirect darkroom lamp. (Krug, American Annual of Pho- — 
I iiss eek a van ete een. y vie'css 40.0.9 'a a eee ee 51 
6 GSP Er 52 
Peveaten darkroom safelight lamp..:...........600. 25 ccc s eee eee eees 52 
Samoavine capinet for plates and films.............-.-.0cccceen ence cctee 57 
Seeewnrincipics Of Tefraction...........2 20sec cee cence eet eee eet eee 60 
29. Refraction in a medium with parallel sides..................0---. 05. 61 
USCS SU 61 
TET Fg ga 62 
memeeiipal forms of simple lenses............. 0.6.00 e stew eee eee ates 62 
33. Image formation: with positive or converging lenses................+. 63 


xi 


XIV ILLUSTRATIONS 
34. Course of light pencils through a negative lens....................-. 63 
35. Image formation according to the Gauss theory...............+-..+- 64 
36. Image formation according to the Gauss theory..................+++-- 64 
37. Position of the nodes in common forms of simple lenses............... 65 
38. Focal length... 0. ..cc sees te ww upc ca ees + +56 6 alneennnnenn nna 66 
39. Table for the calculation of angle of view................sesveeeeee ae 68 
40. Graphic illustration of the principle of depth of focus. (Von Rohr).. 72 
41. Intensity of the optical image. (Brown)....>.9.a290) eee 76 
42. Effect of chromatic aberration on definition.c..7y..9 ee ove 85 
43. Chromatic under correction............ 05... oe sienna 86 
44. Chromatic over correction, ..5........2e2.-0+ 505 sien eee 86 
45. Correction of chromatic correction. (From Beck and Andrews, A 
Simple Treatise on Photographic Lenses) ...........0e.0e-0eusues 87 
46. Irrationality of dispersion. .....3 6.5. ...esess eee) ore ea 88 
47. Spherical aberration... 055... «0+» <u 1 ie eet nae a 90 
48. Spherical aberration in Tessar Ic. (Kellner)....... ive ewe sd oy eee gI 
49. Coma, (Kellner) ...0..06.005 065500 6003 ic pee 92 
50.. Two forms of coma, (Piper), 7.2.0 eae a reo kt pale Pek eee 93 
51. Curvature of field. (Under-correction) (27.72.9522 94 
52. Curvature of field. (Over-correction) .., 7. 2... .a= tee eee 94 
§3. Distortion ov. «te eine ona ee ee ceiey ae ee car iy + oa Nee ee oe, 95 
54. Under and over correction for distortion.) 2.05. oye 96 
55. Constriction of aperture for the marginal pencils. (Brown).......... 97 
56. Greater focal length of marginal pencils resulting in lower intensity. 
(Brown) 2. cea ee ses oie s cine oe oo Blele y ena 97 
57. The angling of the oblique ray. . (Brown) <1... ee oe 98 © 
58. Relation between the angle of view and the diminution vt the optical : 
intensity of the image. (Zschokke)7. 0... ea 99 
59. Astigmatic deformation. ‘(Kellner) .........20009. aa see 100 
60. Astigmatic curves. (Von Rohr) .:..1....... 252 cee IOI 
61. Optical Marea cys trae ie Pee i Ss ae ssa hee ee 103 
62. Wollaston’s meniscus........ 20.000 ss00s5 05 0095 2 105 | 
63. The Chevalier or French landscape lens.... 0.20.4 2a 105 
64. Grubb’s landscape lens.i00 0534.04 ieee 2s ee (fia TOS 
65. Dallmeyer’s rapid landscape lens... .:....... 2. 2 ue et 106 
66. Goddard’s landscape lens—Dallmeyer’s rectilinear landscape lens...... 106 
67. Sutton’s triplet.c. ics. eee Speer Ar eo 108 
68. Dallmeyer’s triple achromatic... 2: ...0... 5 «ees eee ee 108 
69. Harrison and Schnitzer’s Globe lens. °...> ...0. 4 eee ee 108 
70. The Aplanat or R. Row ccc cu wien oc cite oo chen eee ne ene 109 
71. Portrait of Petzval uc oy. ss sans seco 0 viper gee ee Iil 
72. Petzval’s portrait objectivé...... ++. 52. se oan yee III 
73. Modifications of the Petzval portrait objective. ((a@) Dallmeyer, (5) 
Voigtlander, (c) Zinc-Sommer) ...5.29. koe ee Tis 
74. The Goerz Dagor. .. 20. 0esecs ve sas nine ln sien aeons 114 
75, Watson’s Holostigmat............% PP ee 115 
76. The Collinear and Orthostigmat of Voigtlander and Steinheil....... 4 FTO 


Deere ee rOtar oeries VITO. Gi. che de lees ccs ee ceeesccence 117 
TR ERED a he aes alg ley be be ew's Pmt wh able e dad ee see 118 
Meer aNd SYMtOr Of GOETZ. 6.065 ck ck ce eee bea vy ce eee eee e's 118 
ET STE i ge og a 119 
IEC TEE SAEIHOL OCTIVE fo, gsr isis sa Alene eels pu ee dele ee ca eaees 119 
Ea lyicy be sis avis Sv Laake Gc daw dee een bee’ *, ¥120 
nen PE ety ST ISCONTIP MAN. Gok ne Slice Psa bs ces vais cack aveeisavcbaae> I21 
NE 122 
ITO cee se ae RU ds go Mad ie Wie Dv ald Pealev ge ewe e's 122 
nn NPR OCGA ed Mars ke ek ae 8 sie wee be vb elew Javea das 123 
SOU MCR ee ee eles ees ee avn cab ee ced ivee vie 124 
pe eC MME UES. Ses dete eis la el deka evcube dees 125 
mae Ree ANE EDA Ge G's. bc aves eae ode e'vsaceedab lescneaneuus 126 
MP EE SEIDITIR TIC ose sw sine ps oid ees eae dielesceab sue videwenus 126 
Graco eeranoymmetrical Protar i... ci i So lacie cw ceeues 127 
eee 5 Uy dic je Pod oe Obie bad edad ve Huewedeee less 128 
See ER RE PA iy cs ede lo CR vedas lia faa oe bs owe k vaca: 129 
a PME SOG e's, ec Z si coe bs cleus cP ¥ ye bse Oh base a ew Sules 130 
a IS ERIE Pe te, oy Oy a's ee ga ews ba veka eee pew leds 131 
ee IEEE tei ep 2s ee ey es Pee ova skh ea wea edeens 131 
a RINE ET Se ee ol ans db coelp es owl ev dee ead nev Pies 132 
encom. te cenital diveroing lens... ee eee cee cee ce ees 133 
99. Cooke Aviar—shorter foci—longer foci.....................000 05, eta 
ASE CIO MS hk ia ke peo tg a he chee vis de Sec snecacdeccsubye 135 
SS TT) 8 CI ea a 136 
SE, 5 ke ok ie ies Ve edine co dvev ote dehel oboas 136 
ee gs Fk. Das gee bs Vc Sb lad ebb ep whe ek ete’ 137 
See NE CAPO eels fotos ee en Chae edccsgeasuteteaueces 137 
SN Oar Sees yk ge o's Fk as Cea ha bed Oe Coe wee Dae wee 138 
Dia alieevers compound teleobjective.. of. 2.0.00. ee ee cc ek eee 140 
Bare IGOU Ss ee yee ces cic cw kv ac elvan su ueceurvevevesteres I4I 
ROC d Gy Gas tke ele ccc ce tec caer epaewusupeaneuwnas 142 
eae EE Se igs ye pv ci cleans savacauvcsosspwdeianee te 142 
110. Ross Telecentric.......... es ae ere ee eS he Ne Lae, 142 
MUTE SATO AON oe. oc ne blk y oy bb alee dake ka eee use baba we Ge ole . 142 
i EYRE cy he ttle his Cocuc na cas aiea dua waevusoee wees 143 
Ru RT LOTT CCUIV Go. poss vv eo ve Stave nos eb bs hes epee hed awnnrs eee 143 
ee ee ee ccp 66 was ek Cee Mae Ge C4 nding hai ad Role es 144 
I doe suis gad devises oe toes ae pate wa a wihie ad Hedees we hee 144 
Pie oss. 1 ClerOs,.........; OTe Se PII IL REVO See EDR, SER RE AT a 145 
OE VS EUS ES EY) se) en miei i 145 
MM TIIAMIOICG <1 CLEMO YTIAL So suivoace dino C Wise vis Gag od oes es eve dip eeaedeue aes 145 
TEN ey. cg eg moo) Win 4s 9. dw ab elu EON wheal wae a ela eS 146 
120. Emulsion washing apparatus. (From Eder’s Ausfiihrliches Handbuch). 162 
121. Centrifugal separator. (From Eder’s Ausfiihrliches Handbuch)...... 162 
122. The photographic emulsion under a microscope...............000e een, 163 
2 


ILLUSTRATIONS ay 


XV1 ILLUSTRATIONS 


123. Cross section of a developed emulsion. (Courtesy of Dr. A. P. H. 


Trivells) 4s. 0s. vee ooert eee bana 20-3 sl she: Otol cecce ble gee 164 
124. Hodgson’s preferred points... 2... :ss/s5 ss Shuey «lope eee na 166 
125. Size frequency distribution of silver halide grains in a portrait film 

and lantern slide emulsion... 0... 0.40. 5... 20s ene 170 
126. Three-color print of the spectrum...........% + 0s) 172 
127. Visual luminosity of the spectrum after Abney....................... 173 
128. Spectral sensitiveness of the silver halides after Meldola.............. 174 
129. Drying cabinet for sensitized plates..........., .) usu = ena 7 
130. Spectrograph of Eosin. Bureau of Standards paper No. 422.......... 178 


131. Spectrograph of Erythrosine. Bureau of Standards paper No. 422.... 179 
132. Spectrograph of Rose Bengal. Bureau of Standards paper No. 422.... 179 
133. Spectrograph of Orthochrome T. Bureau of Standards paper No. 422 179 


134. Spectrograph of Pinaverdol. Bureau of Standards paper No. 422..... 180 
135. Spectrograph of Pinachrome. Bureau of Standards paper No. 422.... 180 
136. Spectrograph of Pinachrome blue.......,-.7.s..7s0sseeae 181 
137. Spectrograph of Pinachrome violet..:.. 7... 4. seu: 5 seem eee ee 181 
138. Spectrograph of Homocol. Bureau of Standards paper No. 422...... 181 
139. Spectrograph of Pinaflavol. Bureau of Standards paper No. 422...... 182 
. 140. Spectrograph of Cyanine. Bureau of Standards paper No. 422........ 182 

141. Spectrograph of Dicyanine. Bureau of Standards paper No. 422...... 183 
142. Effect of ammonia on sensitizing curve of Dicyanine. Bureau of 

Standards. paper NO, 422.0....4s0a4hienn +» 4's eat had he 184 
143. Spectrograph of Dicyanine A......:..2<. Jig ee 185 
144. Pinacyanol. Bureau of Standards paper ae ABB: 25) side eee 185 
145. Naphthacyanole 2. . 0.56. dss vee ce vale} nals nn ge em ea 186 
146. Kryptocyanine 2.0... 6a. e0' oh ooo. «cum «© wohl cui emnn 186 
147. Red sensitiveness with bisulphite...........-. 0:5. seme en ae 187 
148. Pinachrome and Pinaverdol...........:.... sa 62 ae 188 
149. Action of graduated filters... . 2... 1.15.0 is< owas nee nee 192 
150. Portrait on ordinary and panchromatic emulsions.................-.-- 193 
151. Spectrum of daylight and mazda clear glass bulbs. Bureau of Stand- 

ards paper No. 422. 0.6.24 sens.ss +s ee 4 cis cin pales enn 195 
152a. Photograph of manuscript in blue with red corrections using a green 

111 (0 a CPE 196 
152b. Photograph of manuscript in blue with red corrections showing use 6f 

red filter... 6.0.6 i. on plevs sans ys oe eee ie geen ene 196 
153. Wood sections on ordinary and panchromatic plates. (Courtesy of 

Ilford Ltd.) «0.5 66s Seen sa ee piecic eats poke anen 198 
154. Illustration of the molecular strain theory of the latent image. (Has- 

luck, The Book of Photography); «......:s.<0+ssun ee 219 
155. Chapman Jones plate speed tester. ........... 0s seus) 54s 228 
156. H. and D. sector wheel and exposing apparatus 0.22: 3.400) Gee 229. 
1s7. H. and D. densitometer... 0... 0s. 200s «aus wus «oe ee 230 
158. Illustrating the relation of opacity, transparency ad eae ; ae $6525 
159. The characteristic curve... ois. 565 csv cu eul oes as 55 B36): 2c e 


160. Step chart illustrating the theory of the characteristic curve.......... 237- 


ILLUSTRATIONS Xvi 
161. Characteristic curve secured by crossed wedges...............00..000s 241 
Seem ttittie ant tne Characteristic Curve... 0006. .ee e cceees 244 
163. Development and constant density ratios............ 0.00 ce cee eee eas 245 
Preeremeeiinetty Of gamma. CBrowm)...0..2. 0.0.0. b ieee cee eee ee 249 
rE ele e a oT a kWh bene s easy eee stander ves. 256 
Seems NCTE HIDING 0. a el cele e's See eve ca evi ce ns cecees 257 
eM e Me A Ue. ollie faa Ova Dev dvde de vaabesvecaee 257 
ee er TRITLCOATE hn yen eee decks ven da caedesv cee evadues 258 
eR MEE REI PMIIE RE AND he Ne eg cle sti Ga dele hsb ba bua Gs yeu sane 259 
iene invasion phase of development. (Mees)..............-.00 pene 267 
171. Growth of density with time of development. (Nietz)............... ei. oe 


172. Curve showing growth of density with time of development. ( Nietz).. 273 
173. H. and D. method for calculating the time of development for given 


SN oe clay oxy cia aa uc ceccdcaceestavess 277 
foe ate ctied at Calculating the T.C.... ose. lke eee eee 283 
Reet eee ie Ceveropment Chatt.. .. ee cece ee ee ee penne 284 
176. Effect of a soluble bromide in the developing solution on the plate curve, 285 
177. Density depression with a soluble bromide. (Nietz).................. 286 
178. Effect of soluble bromide on the densities. (Watkins)............... 287 
Poeeoweatunrapig cevyeiopers: (Crabtree)... ...... 2.0... eee eee cee 297 
180. Influence of temperature on time of fixation...............0 0000s sues 340 
181. Influence of concentration of hypo on time of fixation................ 341 
182. Influence of ammonium chloride on the time of fixation............... 343 
pert GET ISIGIIN SAONATATUS. 0.022... cee ec ce cs cae ene te ee cue nee 354 
enn EE EOL FOU IM... ec ec cc ee vee ee bree ted etedaewene 355 
ee SO ils ea ce pices cin vAsley nis caged anwscesawe fees 356 
186. Sensitometric action of different reducers. (Nietz and Huse)........ cee: 
187. Action of a proportional reducer on the plate curve. (Nietz and Huse), 376 
188. Sensitometry of photographic intensification. (Nietz and Huse)...... 387 
189. Bench for local reduction. (British Journal of Photography)........ 389 
190a@, 190b, 191. Adapting the scale of the printing paper to the negative.... 391 
192. Printing machines for amateur and for professional use.............. 397 
193. Effect of time of development upon the characteristic curve of paper 

SMSIONS) 5... es 8s PN ara Ale Sle Pe nse Re SO tg gto pee ee ¢ 401 
ieee reetnicay aperated print» washer, (Pako) ....... 000.2000 e cee eee ces 404 
195. Centrifugal water pressure type of print washer. (Halldorsen)...... 405 
RR PRN EE P(SICKIC) 55. oa fo a cee sas wane vd pena ce sees 406 
eet PrOLection PLINting 2... ee ee ee sas eee bee hen ee ce ns 415 
er T Ak. kw Vdc do ke Caw Gad ebunaceeseuecanuheye 416 
Pee SD eMIAT OAT OCAMCTA ie ck ce ns ees eh eae eho cu sea sawe we ebe wee 417 
200. Projection printing apparatus for use with daylight.................. A418 
pomeaierene slaern for artificial light. ... 0.6.6 eee ne ee ee ee ee we 419 
202. Proper position of the carbons of an arc light for use with alternating 

aa are aC, Nn in Spb, vk Bane mel SR RV ee eee wie a wpa al slo 8s, oh 421 
203. Acetylene burner for projection printing. (Burke and James)........ 422 
204. Lamps burning methylated spirit for projection purposes.............. 423 


205. Parallax reflector for use with incandescent electric sources......... ee 


XVili - ILLUSTRATIONS 


206. 
207. 
208. 
200. 
210. 
ail. 


212. 


213. 
214. 
215. 
216. 
ai7, 
218. 
210. 
220. 


221. 
222. 
223, 


224. 
225. 
226. 


227. 


228. 


220. 
230. 
231. 
232. 
233. 


234. 


235. 
236. 
237. 
238. 


Securing even illumination with five incandescent electric sources...... 442 
Forms for the lighthouse using reflected light. (Wall).............. 425 
The function of the condenser...... «0. <:./.:: a0. +s aisle en 426 
Conjugate foci in enlarging... .. i... 2s 5 «be eae 427 
Adjustment of the light source with condensers..... Pere ere eT 428 
Loss of light between condensers due to the use of a long focus lens for 
projection. ©» (Candy) .....4.45.55+ 00 eanw Pee nee 430 
Loss of covering power owing to the use of ae: focus projecting lens | 
with condensers. (Candy) ......... 00%. «05s Gee stnunnnnnnnanE 431 
Ingento enlarging easel for use with printing frame................... 432 
Westminister enlarging easel...... ww des'e0:0 lal oie rn 5 ea ie 
Scatter of light by negatives. (Callies) . <4 ccm 434 
Graduating focusing scale for enlarging. (Lockett)................. 438 
F and S Lantern slide printing frame... «..<..-. seen ane meee 446 
Century lantern slide camera for reduction.«..:... «2+. 55. 448 
Slide making by reduction using daylight... ...........:+.+ss eee 448 
Device for holding lantern plate in position when using enlarger for 
lantern slide making by reduction. (Charles).................... 449 
Actinometers for carbon printing... .....<.+¢ec se eaeh eee 498 
Squeegee board for carbro. (Farmer) ....20. aq ane 506 
Curves showing the influence on contrast of variations in proportions 
of gum, pigment and sensitizer in the gum-bichromate process. 
(Anderson) | 4 a6 0c b0 5 ok boss aa fiw ain Mine Goorin een 514 
Owens’ frame for multiple printing. ..<...<.s<-s-;+shese nee eee 517 
Zerbe’s method of registration.<. .....=-«:<+ «srs on eel 
Proper position of the brush in pigmenting. (Mortimer and Coult- 
hurst, The Oil and Bromotl Processés) 4.5.4. .-4- ee 530 
Results in pigmenting. (Partington) ...... 9.2.45 essen 541 
Transfer presses... ..ccce0 06 vue on se ose o's su yee gig tens gneiss nnn 546 
Prett’s transfer press... .... 0.550200 se% ss 5 6 ogee Cu cee 547 
Copying stand. .......00c0 see ben vececccuw ss epeiie pene Eten nnn 
Book holder for copying... 0... 0.06.05 «+0 sue sn au Opies 551 
Illumination of the copy using daylight........0.. sae ae seen 553 
Copying apparatus for artificial light. (Rose, The Commercial Pho- 
tographer). 60. vcaceakeen utius pa 0 duiphe 5 Soke geen een 553 
Method of securing white or black ge (Photo. courtesy of 
D. J. Pratt) <6. cs onic 0% a igo bonis aoa ta gee eee 567 
Sanger Shepherd three-color camera, ...:... 2. «-.4s084 05 eee ve ae 
Butler’s one-exposure, three-color camera.............-....4++<sbeeem 572 — 
Autochrome screen, X 125........-ce reece ree ane Te: 578 
Duplex Screen, oie. oes ca cnet ba 8 057% cue k 5 asnenien ane n .. 585 


CHAPTER IT 
THE DEVELOPMENT OF PHOTOGRAPHY 


Introduction.—Photography is the science of obtaining images 
of objects by the action of light on sensitive substances. The word 
photography is due probably to Sir John Herschel and ts derived 
from two Greek words (¢%s = light, ypadu = write) meaning to 
draw by light. 

The two sciences of optics and chemistry form the basis of pho- 
tography, the former being concerned with the production of the 
image in the camera, the latter with the composition and treatment of 
the sensitive surface which reproduces the image cast upon it by the 
lens. 

The Development of the Camera.—The invention and development 
of the camera forms a most important phase in the history of pho- 
tography. Camera is a Latin word originally meaning an enclosure 
with a vaulted, or arched, cover, but in course of time came to mean 
a room. Thus the camera obscura means a dark room, except for 
the illumination which comes through the small opening which serves 
as the lens. (Fig. 1.) We do not know by whom, or at what date, 
the principle of the camera obscura was first discovered. There was, 
properly speaking, no invention of the camera obscura, for the prin- 
ciple is a natural phenomenon which must certainly have been ob- 
served many times by man without having excited any particular 
interest until someone more enterprising than the rest set about to 
find the cause of the phenomenon and its possible applications. At 
whatever date this may have taken place, we have at least a reference 
to the principle of the camera obscura as early as the time of Aris- 
totle. This learned Greek in his Problemata published about 350 B.C. 
refers to the fact that the image of the sun formed by the rays of light 
passing through a square aperture appears circular. He also noted 
the amplification of the image as the distance from the aperture is 
increased, Even a man with his intellect, however, appears to have 
been unimpressed, so that the camera obscura, properly speaking, es- 
caped him and he has really no place in its history. 

1 


2 PHOTOGRAPHY 


From the time of Aristotle there is no mention of the camera 
obscura for many hundred years. Alhazen in his Thesaurus Optice 
written in the eleventh century, although not published until 1572, 
seems to have been more or less familiar with its principle although 
he does not mention it specifically. Roger Bacon in his Perspectiwa, 
published in 1267, has a passage which many have taken as the first 
description of the camera obscura, but it is so indefinite that it may 


(Courtesy of the Smithsonian Institution.) 


Fic. 1. The Camera Obscura from an Old Print. 


be equally as well regarded as applying to the projection of images. 
There are other passages in his Opus Majus which seem to indicate a 
knowledge of the principles of the camera obscura but these likewise ~ 
are couched in such vague terms that we are hardly justified in 
crediting him with the discovery of the camera obscura. 

The first precise and complete account of the camera obscura is to 
be found in one of the unpublished manuscripts of Leonardo da 
Vinci quoted by Venturi in his Essai sur les Ouvrages physico- 
mathematiques de Leonardo da Vinci which was published at Paris 
in 1797. The following is Venturi’s translation of the passage in the 
works of Leonardo: : 

The following experiment shows how objects send their images to intersect 
on the albiginous humor inside the eye. When the images of illuminated objects 


enter into a very dark chamber by a small round aperture, if you receive these 
images in the interior of the room on a piece of white paper placed at some dis- 


Sag has 


ta oe ele 


THE DEVELOPMENT OF PHOTOGRAPHY 3 


tance from the aperture, you will notice on the paper all the objects in their 
proper forms and colors: they will be lessened in size and will be reversed, and 
that in virtue of the intersection already noticed. If the images come from a 
place lit by the sun, they will appear as if painted on the paper, which should be 
very thin and looked at from behind. The aperture should be made in a very 


thin piece of sheet iron. 


Leonardo then goes on to give a diagram showing the arrange- 
ment of the aperture and screen and the course of the light rays. The 
manuscript is undated, but as Leonardo died in 1519 it probably dates 
from several years previously. It is noteworthy that he does not 
refer to it in any way as an invention, which would lead one to be- 
lieve that he was not sure of having been the first to describe the safne. 

In Cesariano’s translation of Vitruvivius’ Treatise on Architecture, 
published at Como in 1521, there is a passage referring to the camera 
obscura as having been discovered by a Benedictine monk, Don Pap- 
nuito. Libri in his Historie des sciences mathematiques en Italie 
(Paris, 1841) says that although this is the first published description 
of the camera obscura, the observations of Leonardo must certainly 
have been made at an earlier date. 

Maurolycus, an eminent mathematician of Messina, in his Photismi 
de Lumine et Umbra ad Perspectivam et radiorum incedentiam fa- 
cientia, published in 1611, but finished in 1521, treats the subject 
mathematically and gives several theorems relating to the passage of 
light through small apertures and was apparently well acquainted 
with the formation of images in this way. 

The next references to the camera obscura are found in Germany, 
where we find Erasmus Reinhold and his pupils Gemma Frisius and 
others using the same to observe eclipses of the sun without danger 
to the eyes. Reinhold probably used the camera obscura in this way 
as early as 1540. ; 

Jean Baptiste Porta——The connection of Jean Baptiste Porta with 
the discovery of the camera obscura arises from a passage in his 
Magia Naturalis, a remarkable work published in Naples, 1553, when 
he was fifteen years of age. Although there are at least five accurate 
and precise descriptions of the camera obscura prior to the time of 
Porta, still he is popularly credited with its discovery. It is quite 
likely that this misconception arose from the fact that Arago, the 
eminent secretary of the French Academy of Sciences, in his address 
before that body on the occasion of the presentation of the details of 


4 » PHOTOGRAPH} 


the Daguerreotype process took the opportunity to make a few re- 
marks on the historical phases of the subject in which he referred to 
the work of Porta with the camera obscura. There is nothing to 
show that Arago had investigated the subject thoroughly and, al- 
though he did not distinctly credit Porta with the discovery of the 
camera obscura, the prominence given him by one of Arago’s emi- 
nence established him in the popular mind as the inventor of the 
camera obscura: As a matter of fact, Libri, a colleague of Arago, 
in a work on the history of the mathematical sciences in Italy, already 
referred to, called attention to the work of Leonardo and several 
others who anticipated Porta in the discovery of the camera obscura. 
This work, however, apparently never reached the photographic 
fraternity, with the result that year after year the writers of text- 
books in referring to Porta as the inventor of the camera obscura 
firmly established the myth in the popular mind and it was not until 
the appearance of Dr. Eder’s Geschichte der Photographie, and the 
work of Waterhouse, that the work of Porta’s predecessors was 
properly brought before the photographic world.* 

The Camera Obscura with Lens.—The first definite description of 

a camera obscura with a lens is found in a work on perspective, La 
Pratica della Prospectiva, by a Venetian nobleman, Daniello Barbaro, 
which was published at Naples in 1568. In 1585, seventeen years 
after the appearance of this work and four years prior to the ap- 
pearance of the second edition of Porta’s Magia Naturalis, in which 
the description of the camera obscura with lens occurs, another 
Venetian nobleman, Giovanni Battista Benedetti, in a book of mathe- 
matical and physical observations published at Turin, again refers 

to the camera obscura with lens. 

The lens used by Barbaro and Benedetti was planoconvex in form. 
Kepler, who took up the study of the camera about the beginning 
‘of the seventeenth century and investigated it thoroughly both theo- 
retically and practically, was the first to perceive the advantage of a 
compound objective composed of concave and convex lenses. In his 
Dioptrice published in 1611, he deals with the principles of refraction, 
of image formation, and the properties of various forms of lenses 
and their combinations. He speaks of the disadvantages of the plano- 
convex form as regards the small field and the advantages of the use 


1 For a biography of Porta and an account of his predecessors see Bull. Soc. 
franc. Phot., 1923, p. 52. 


THE DEVELOPMENT OF PHOTOGRAPHY 9) 


of a concave lens with the convex. Kepler really did a great deal 
towards placing the optics of the camera on a firm foundation, a 
branch of his work which has not received the attention which it de- — 
serves. 

Early Records of the Photochemical Action of Light.—The tan- 
ning of the skin by light is one of the many common evidences of the 
action of light which could hardly escape the attention of man, even 
in the savage state, but this action is so slow that it is not sufficiently 
striking to excite more than casual interest. More than 300 years 
before Christ, Aristotle observed that the green color of plants was 
due to light, for plants which had become bleached in the dark turned 
green again on exposure to light. The first observation on record of 
the action of the atmosphere on silver is by Pliny about 100 A.D., but 
his observations are probably only a record of the action of the at- 
mosphere on metallic silver. In the eighth century, however, Jabir 
Ibn Hayyam, often called Geber, observed the darkening of silver 
‘nitrate on exposure to atmospheric action. | 
In 1556 Georgius Fabricus recorded the fact that crude silver 
chloride, or horn silver, an ore frequently found in the mines of 
Frieburg, darkens on exposure, and Boyle, an early English chemist, 
writing about 1686 speaks of the sensitiveness of gold. 

In 1725 a Russian field marshal prepared a remedy in which ferric 
chloride is used, using the action of light to reduce it to the ferrous 
state. 

That the darkening of the silver salts is due to light and not to 
the action of various vapors of the atmosphere was first definitely re- 
corded by Johann Henrich Schulze in 1727. .(Fig. 2.) While ex- 
perimenting at an open window with a solution of chalk and aqua 
regia, which accidentally contained a trace of silver, he was surprised 
to find that the surface of the solution which was exposed to light 
had changed to a dark purple color, while the body of the solution re- 
moved from the light had not changed. Following up his observa- 
tions, he made a fresh solution of chalk and aqua regia, which he 
exposed to light under precisely the same conditions as the first so- 
lution. As this mixture was unaffected, he rightly concluded that 
the sensitiveness of the first solution had been due to the trace of 
silver. Cutting a stencil in opaque paper he placed the same around 
a bottle containing some of the mixture and exposed it to light. In 
this way the words or sentences were accurately and distinctly re- 


6 PHOTOGRAPHY 


produced on the chalk sediment and the result was looked upon as 
a marvel by ignorant people. 


MGkbhahSSReLLORURL CGH SIL ELL IETELSCe et ccUSE eK t 


Fic. 2. Johann Heinrich Schulze. (From Eder’s biography) 


While Schulze was undoubtedly the first to secure an image by the 
agency of light, his work, although far in advance of his time, is 
hardly sufficient to earn for him the title “ Discoverer of Photogra- 
phy ” such as has been given him by Eder and others. The work of 
Schulze was a great advance, and we are certain a great stimulus to 
later work along similar lines, but as he made no attempt to use the 
camera obscura, nor to “ fix” the image which he obtained, it seems 
hardly fair that he should be termed “ Discoverer of Photography.” * 

In 1763, Dr. William Lewis published an account of his investi- 
gations on the cause of the discoloration of bone, ivory, wood, etc., 
when treated with silver nitrate and exposed to light, and in his His- 
tory of Discoveries Relating to Light, Vision and Color, Dr. Joseph 
Priestley refers to the previous work of Schulze and Lewis. The 
principal interest, however, which we have in this work of Priestley 

2 For an interesting biography of Schulze see Eder, Johann Heinrich Schulze— 
Der Lebenslauf des Erfinders des Ersten Photographischen Verfahrens und des 
Begrunders der Greschite der Medizen. Wien, 1920. 


A full translation of Schulze’s paper describing his researches with silver salts 
appeared in the Photographic Journal for 1898, page 53. 


THE DEVELOPMENT OF PHOTOGRAPHY ie 


is its connection with a later experimenter, Thomas Wedgwood, who 
without doubt was led to the subject through its pages. 

In 1777, Carl William Scheele noted the influence of various colors 
on the rate of darkening and found that blue and violet light were 
much more active in darkening silver nitrate than red or orange. 
Scheele also investigated the chemical changes involved in the darken- 
ing of silver chloride and discovered that the effect of light on this 
substance is to cause the evolution of chlorine. 

Herschel in 1800 discovered the heat rays beyond the visible red, 
and the following year Ritter discovered, by photographic means, 
the existence of the very active ultra-violet beyond the visible violet. 


Fic. 3. Thomas Wedgwood. (Chalk drawing, author unknown) 


The Forerunners—Wedgwood and Davy.—Neglecting the work 
of Boulton in 1777, Charles in 1780, and Lord Brougham in 1795, 
whose claims to the previous discovery of photography are too vague 
to be seriously considered, we arrive at the important work of Wedg- 


8 : PHOTOGRAPHY 


wood, who made the first definite step towards the discovery of pho- 
tography. 

_ Thomas Wedgwood, fourth son of the great potter Josiah Wedg- 
wood, was born the fourteenth of May 1771. (Fig. 3.) On account 
of his delicate health, most of his education was conducted at home 
and he had as tutor a Mr. Alexander Chisolm, who had formerly 
acted as secretary to Dr. William Lewis and from whom Wedgwood 
was no doubt able to learn of the work of Schulze and the others who 
had preceded him. 

Wedgewood, together with Humphrey Davy, then a rising young 
chemist, repeated the work of Schulze with silver nitrate and were 
able to make prints of leaves, and similar objects, but were unable to 
prepare a paper sufficiently rapid to permit of its use in the camera 
obscura. Davy made some important additions to the work of 
Schulze. He found that silver chloride was more sensitive than the 
nitrate, and using the concentrated light of a solar microscope, he 
was able to secure images of small stationary objects on his silver 
chloride paper. But neither Wedgwood nor Davy were able to find © 
a means of “ fixing ” the image, or dissolving the unacted-upon silver 
salt so as to render the image permanent. The poor health of Wedg- 
wood was no doubt partly responsible for this, and the whole subject 
was abruptly terminated by his death at the early age of thirty-one 
years. After his death, a joint paper, written probably by Davy, was 
brought before the Royal Institution and appeared in the Journal for 
1802 under the following title: An Account of a Method of Copymg 
Paintings upon Glass, and of Making Profiles by the Agency of Light 
on Nitrate of Silver, by T. Wedgwood with observations by H. Davy. 

Davy does not appear to have paid any attention to the subject 
after the death of Wedgwood and no further work on photography 
was done in England until the researches of Talbot beginning about 
1835. While the work of Talbot logically follows that of Wedgwood 
on account of the close similarity of the methods adopted by the two 
experimenters, we must first consider the important work of Niepce — 
in France, whose researches began about 1812. 

The Life and Work of Joseph Nicephore Niepce.—Joseph Nice- 
phore Niepce (Fig. 4), the first man to obtain a permanent photo- 
graph, was born at Chalons-sur-Saone on March 7, 1765. His father 
was a man of means and Nicephore and his brother, Claude, had a 
tutor in language and science in which both early showed especial 


THE DEVELOPMENT OF PHOTOGRAPHY 9 


interest. Designed for the Church by his parents, the Revolution 
upset his plans, and Nicephore joined the army in 1792 and served two 
years in Italy, when ill health compelled him to resign his commission 


(Courlesy of the Société Francaise de Pholographie). 
Fic. 4. Joseph Nicephore Niepce 


and return to his country estate, where, having married, he spent the 
remainder of his long life in scientific pursuits, of which photography 
was by no means the least. His brother Claude, to whom he was 
devotedly attached, resided with him until 1811, when, in order to 
further his scientific work, he left for Paris and finally to Kew in 
England. Unfortunately Niepce left no written account of his work 
and our only source of information is from his correspondence to 
his brother Claude. In 1827 Nicephore visited his brother Claude in 
England and brought with him some prints and a paper which he 
hoped to bring before the Royal Society, but having refused to make 
public the methods employed for ‘making the prints, the rules of 
the Society compelled them to refuse the communication. 

The same year he met Daguerre in Paris and after overtures lasting 


10 


PHOTOGRAPHY 


Fic. 4a. Birthplace of Niepce 


Fic. 4b. Statue to Niepce 


THE DEVELOPMENT OF PHOTOGRAPHY 1] 


two years the two investigators signed articles of partnership to con- 
tinue for ten years, during which time the two would work to their 
joint advantage. Niepce made no further advance on his process 
after this date and on his death in 1833, in his sixty-eighth year, his 
son Isidore succeeded him in the partnership. 


Fic. 5. A Heliographic Print by Niepce, 1824 


The basis of the process worked out by Niepce was the discovery 
that bitumen of Judea or “ Jews’ pitch” becomes insoluble upon ex- 
posure to light. Niepce dissolved bitumen of Judea in oil of lav- 
ender and spread a thin layer on stone or metal plates. The sensi- 
tized plate was then exposed for several hours under the transparent 
drawing to be reproduced, after which the plate was immersed in 
oil of lavender which dissolved the parts unaltered by light, leaving 
the plate bare in these places and accurately reproducing the outlines 
of the drawing. By treating the metal plate with acid, an image in 
relief was produced from which prints could be secured in an ordi- 
nary printing press. One of these early prints, dating from 1826, is 
the Cardinal plate, illustrated in Fig. 5. 

A letter of Niepce recently discovered by G. Cromer ® shows that 
Niepce was successful in the use of the camera obscura as early as 
1826. The letter describes what is perhaps the first permanent re- 


3 Bull. Soc. franc. Phot., 1922, p. 60. 


12 PHOTOGRAPHY 


production of a natural object by photographic means. The time of 
exposure required was from six to eight hours, so that, while Niepce 
may be said to have discovered photography, his process was of little 
practical value. Nevertheless we must not lose sight of the fact that, 
although far from perfect, his process was the first by which a 
permanent reproduction might be secured by photographic means and, 
furthermore, he was the first to successfully make use of the camera 
obscura, so that to Niepce belongs a large share in the discovery of 
photography. 


DAGUERAS 
Gas fess 


ER 


Fic. 6. Louis Jacques Mande Daguerre 


The Life and Work of Jacques Mande Daguerre.—Jacques Mande 
Daguerre (Fig. 6), who invented the Daguerreotype, the first practi- 
cal process of photography, was born at Cormeilles, a small village — 


THE DEVELOPMENT OF PHOTOGRAPHY 13 


about ten miles from Paris, on November 18, 1787. His father was 
court crier of the village and his mother from one of the village fam- 
ilies. During the Revolution his father lost his position and moved 
to Orleans, where the young Daguerre grew up. He was educated in 
the public schools of France and early showed especial aptitude for 
drawing ; producing, it is said, creditable portraits of his parents and 
friends at the early age of thirteen. At the age of sixteen, he left 
Orleans to begin life in Paris. There he finally secured employment 
with Degotti, a flourishing scene painter, and at this the young 
Daguerre made rapid progress, soon equalling, and finally excelling 
his master, so that his services were much sought after by the lead- 
ing theaters. During the years 1816 to 1821 he assisted Pierre Pre- 
vost with his panoramic paintings of the cities of Europe and during 
this time it is probable that he first became acquainted with the camera 
obscura. 

In the production of the large paintings required for the diorama, 
which he opened in Paris in either 1822 or 1823, Daguerre frequently 
made use of the camera obscura and it was the remarkable beauty 
and perfection of the image produced by this instrument that led him 
to attempt to find a way by which the image might be made perma- 
nent. His investigations seem to have begun about 1824. Two years 
later he received word, probably from Chevalier, an optician from 
whom he had been in the habit of purchasing the apparatus necessary 
for his experiments, that the subject was also occupying the attention 
of a man in the Provinces, Joseph Nicephore Niepce. Daguerre im- 
mediately wrote to Niepce suggesting an exchange of secrets, but let- 
ters to Niepce received but curt replies until 1827 when Niepce was 
called to England on account of the serious illness of his brother 
Claude. Stopping in Paris he met Daguerre and cordial relations 
were established between the two investigators. On December 5, 
1829, the two workers signed an agreement of partnership to continue 
for ten years, during which time each would work to their mutual 
advantage. After the death of Niepce in 1833, if not before, 
Daguerre discarded the method of the elder investigator and started 
out on different lines. In 1835 he informed Isidore Niepce, who had 
succeeded to his father’s interest in the partnership, that he had 
reached a certain amount of sucess, and after two years more of 
perfecting details, a company was formed to buy out the process for 
the sum of 200,000 francs. This, however, was a failure and in their 
extremity the two were forced to appeal to the Government. 

3 


14 


PHOTOGRAPHY 


CHA: 


(Courtesy of the Société Frangaise de Photographie) 


Tomb of Daguerre. 


Fic. 7. 


THE DEVELOPMENT OF PHOTOGRAPHY 15 


Daguerre showed specimens, and placed a written account of his 
process in the hands of Arago, the eminent physicist and astronomer, 
in January 1839. Arago was impressed with the possibilities of the 
process and brought the matter to the attention of the Home Minister, 
to whom Arago’s endorsement was sufficient, and on his recommenda- 
tion the Government awarded Daguerre a life pension of 6,000 francs 
yearly and to Isidore Niepce one of 4,000, on the condition that the in- 
vention be published without patent, this money being paid by France 
“for the glory of endowing the world of science and of art with 
one of the most surprising discoveries that honor their native land,” 
to quote the official document. The stipulations were agreed to and 
in August of the same year (1839) the details of the process were 
made public before the Academie des Sciences. Interest in the process 
spread rapidly and the inventor made a small fortune in the sale of 
apparatus for the process. Daguerre died at Petit-sur-Marne in 
1851 at the age of sixty-three, having lived to see the science take a 
large and important place in the affairs of the world. (Fig. 7.) 

The Daguerreotype Process.—The Daguerreotype process occu- 
pies such an important place in the history of photography that an 
extended description of the same will not be out of place. 

A silver plate, or copper plate covered with silver, is rubbed with 
tripoli and olive oil and polished with rouge and cotton wool to ob- 
tain a highly polished, perfectly smooth, faultless surface. This 
polished plate is pee with its polished side down in a fuming 


WA 


~\ 


N ~& 

\ WA 
Fic. 8. Fuming Cabinet for the Daguerreotype Process 

cabinet (illustrated in Fig. 8) on the bottom of which is a thin layer 


of iodine crystals. As the iodine evaporates the vapors come in con- 
tact with the silver and form silver iodide. After passing through 


16 PHOTOGRAPHY 


several successive changes of color, the surface of the silver plate 
becomes blue and when this stage is reached the plate is removed, 
placed in the holder, and exposed. Exposure with such a plate for 
three to four hours produces only a faint impression of the silver 
iodide and if it had not been for the accidental discovery of the latent 
image, and the possibility of developing the image by chemical means, 
Daguerre would have fared no better that his predecessors. For- 
tunately, however, Daguerre discovered that mercury had the power 
of bringing out the visible image, so that the exposure necessary was 
shortened to three or four minutes. The discovery of the latent 
image, and the possibility of developing the same, was the greatest 
step towards the realization of practical photography made by 
Daguerre and one which must forever entitle him to a high place 
among those who have contributed to the advancement of the science. 

Development is conducted in a developing box (illustrated in Fig. 


Fic. 9. Developing Box for the Daguerreotype Process 


9g). The heat of the spirit lamp under the dish of mercury causes 
the mercury to condense on those parts of the image affected by ex- 
posure to light and the image gradually develops as more and more 
mercury is deposited until a complete reproduction of the original is 
obtained. For fixing, Daguerre at first used common salt, but soon 
after the publication of the process Herschel called attention to the 


use of “hypo” which was immediately adopted.* 


4 The chemical reactions involved in the Daguerreotype process cannot be said 
to be fully understood even at the present time. For a very complete discus- 
sion of the same see Waterhouse, “ Lessons from the Daguerreotype,” Photo. J., 
1899, 39, 60, and 1898, 38, 45. 


THE DEVELOPMENT OF PHOTOGRAPHY 17 


Later History of the Daguerreotype.—Daguerre’s early plates re- 
quired an exposure of three to four minutes but the following year 
(1840) a London science lecturer, Mr. Goddard, discovered the fact 
that a combination of bromine and iodine was much more sensitive 
than either alone and a great increase in rapidity was secured; which 
was still further increased by the introduction of the really wonder- 
ful Petzval portrait lens by Voigtlander the following year. 

The first attempts at portraiture appear to have been made in 


America. The claims of Dr. J. W. Draper and Robert Cornelius of 


Philadelphia to have made the first human portrait by photography 
have been reviewed by L. T. W. in the British Journal of Photog- 


raphy. It appears that the portrait of his sister was made by Dr. 


Draper on March 31, 1840, while Robert Cornelius opened a studio 
for the Daguerreotype process in Philadelphia on February 18, 1840. 
As Cornelius must certainly have made experiments before opening 
a studio professionally it appears that he, and not Draper, was the 
first to make a portrait by the Daguerreotype process. Draper’s por- 


ia 


ek Mek 
Fic. 10. An Early Daguerreotype Portrait. 
Often stated to be the first portrait by photography 


trait, reproduced in Fig. 10, is the first Daguerreotype portrait which 
is nOw in existence, none of the results of Cornelius being in existence, 
so far as known.? 


5 Brit. J. Phot., 1920, 67, p. 420. 


18 PHOTOGRAPHY 


The Daguerreotype process lasted only about ten years or until the 
discovery of the wet collodion process by Scott-Archer in 1851. It 
was almost entirely a portrait process and was not used for landscape 
and other exterior work to any extent. 

The Positive Process of Bayard.—Bayard, one of the founders of 
the Société Francaise de Photographie, demands a few lines for the 
positive process which he worked out prior to the announcement of 
the Daguerreotype. On June 24, 1839, two months before the details 
of the Daguerreotype process were made public, Bayard exhibited 
a collection of silver prints made by a method entirely different from 
that employed by any of the early workers. His process produced 
a positive print direct and without development. Paper was soaked 
in a solution of ammonium chloride, dried, floated on silver nitrate 
and after drying in the dark it was exposed to daylight until com- 
pletely darkened. Before exposure, it was placed in a solution of 
potassium iodide and exposed while wet in the camera. The action 
of light bleached the paper, producing a positive result which was 
washed and fixed in potassium bromide. The prints were permanent 
and some are said to be in existence at the present day. 

The Life and Early Work of William Henry Fox-Talbot.—Wil- 
liam Henry Fox-Talbot (Fig. 11) was born February 11, 1800, at 
the ancestral home of the Talbots, Lacock Abbey, in Wiltshire. He 
was of an old and well-established family, the Talbots ranking among 
the oldest families in England, while his mother was a daughter of 
the Earl of Ilchester. He was educated at Harrow and Cambridge, 
leaving the University in 1821 with highest honors, and for two years 
was a member of parliament, but politics did not interest him and he 
retired in 1835 to devote the remainder of his life to science. Talbot 
was a versatile experimentalist; his earlier years were devoted to 
photography, but in later years he wrote on a wide range of subjects, 


as spectrum analysis, inscriptions in Egypt, the optical phenomena 


of crystals and integral calculus, while apparatus in the memorial 
collection at the Royal Photographic Society speak of his interest in 
electrical and physical science. 

Talbot relates in his Pencil of Nature, published in London 1844, 
that in 1833 he was sketching on the shores of Lake Como in Italy 
with the camera obscura, but without much success owing to his lack 
of knowledge of drawing. On his return to England in January the 
year following he determined to follow up the work of Schulze and 


a ee - ; 
EES en ee — 


— 


THE DEVELOPMENT OF PHOTOGRAPHY 19 


Wedgwood on the action of light on silver salts. His first experi- 
ments with silver nitrate and silver chloride were unsuccessful, as the 
paper was not sufficiently sensitive to light. As a result of many 
trials, Talbot found that a far greater degree of sensitiveness was 
obtairied with silver chloride if a weak solution of salt was employed, 


Fic. 11. William Henry Fox-Talbot 


producing what he termed an “ imperfect ” chloride, which was very 
much more sensitive to light. Using paper prepared in this manner 
he was able to readily obtain prints of tracings, leaves, etc., as had 
Wedgwood and Davy before him and with whose work he was fa- 
miliar. ‘Talbot, however, was successful where earlier investigators 
had failed; he found that a solution of common salt would dissolve 
the unacted-upon salts and render the image permanent. In 1835 he 
found that the sensitiveness of his paper was greatly increased by 
giving it successive washings in salt and silver and exposing it 
while still wet. With paper so prepared he made a picture of his 
home, Lacock Abbey, using the camera obscura during this same year 
(1835). 


20 PHOTOGRAPHY 


The details of Talbot’s process, which the inventor styled “ Photo- 
genic Drawing,’ were first made public in a communication to the 
Royal Institution by Faraday on January 25, 1839, and a week later 

- (January 31, 1839) Talbot himself read a paper on the subject before 
the Royal Society, of which he was a member. This was Talbot’s 
first paper, and it will be observed that it was published almost eight 
months before the details of the Daguerreotype process were made 
public. 

The Calotype Process.—The principal work of Talbot, however, 
was the Calotype process invented by him in 1840. In this process 
silver iodide was used, the paper being impregnated with silver iodide 
and immediately before exposure was washed over with a mixture of 
gallic acid and silver nitrate. After an exposure of about a minute, 
the image was developed in gallic acid and silver nitrate. Talbot had 
at last grasped the idea of a developer for bringing out the latent 
image. After fixing and drying, the paper negative was placed over 
a similar sheet, of sensitized paper and exposed to obtain the positive 
proof. The process was fully described by Talbot before the Royal 
Society on June 10, 1841. 

It is practically certain that in the use of gallic acid to increase the 
sensitiveness of his paper, and also as a developer, Talbot had been 
anticipated by an English clergyman, Rev. J. B. Reade, but as the 
latter did not publish an account of his work, Talbot’s discovery was 
independent and original with him. 

Miscellaneous Paper Processes.—The work of Talbot was soon 
estimated at its true value by a French investigator, Blanquart-Evard, 


A process devised by him was practically identical with that of Talbot 


except for the employment of silver chloride instead of iodide. Nu- 
merous other processes were invented by various workers, none of 
which are of more than historical interest, and these we will merely 
mention, referring those who may desire further information to 
Robert Hunt’s Treatise on Photography published in London 1854. 


Amphitype—invented by Sir John Herschel. 

_ Anthotype—invented by Sir John Herschel. 

Catalysotype—invented by a Dr. Wood. 

Chromatype—process using chromic acid. 

Chrysotype—invented by Sir John Herschel. 

Energiatype—invented by Robert Hunt. 

Fluortype—so called from use of salts of fluoric acid invented by 
Robert Hunt. 


en ee a a ee 


a ee a a oe 
2 


THE DEVELOPMENT OF PHOTOGRAPHY 21 


Application of Daguerreotype process to paper by R. Hunt. (See 
Treatise on Photography, 1854, p. 85.) 

The Introduction of Glass——Paper is not an ideal medium for 
negatives owing to its relatively coarse grain, which destroys fine de- 
tail, and its opacity, which makes printing slow. To overcome these 
difficulties Sir John Herschel early attempted to substitute glass, but 
his process was unsuccessful because he did not recognize that images 
of sufficient opacity can be obtained only in the presence of albumen, 
gelatine, or some similar substance, which is capable of attracting and 
combining with the silver salts. The need of such a substance was 
recognized by Niepce de Saint Victor, a nephew of Nicephore, who 
was responsible in 1847 for a method in which albumen was used. 
The white of an egg was beaten up with potassium iodide and com- 
mon salt and the clear liquid poured over a glass plate and allowed to 


dry. In this state the plates could be kept for some time. Im- 


mediately before exposure the plate was dipped in a bath of silver 
nitrate, which caused the formation of a sensitive silver-chloro-iodide 
within the pores of the albumen. The plate was exposed either wet 
or dry and developed in gallic acid. Although no gain in rapidity was 
made by the albumen process, the results were much clearer and the 
negatives printed more rapidly, so that the process was immensely 
popular until the introduction of collodion. 

Scott-Archer and the Introduction of Collodion.—In 1847 Schon- 
bein and Bottcher discovered gun cotton and the following year 
Maynard of Boston showed that the same might be dissolved in a 
mixture of alcohol and ether to produce a substance of a viscid na- 
ture which is termed collodion. In 1849 Le Gray, a French investi- 
gator, suggested the use of collodion in photography and in a book 
published in 1850 Robert Bingham, assistant to Faraday, suggests the 
use of collodion in place of albumen, but the credit for the invention 
and publication of a workable process employing collodion is due to 
Frederick Scott-Archer. (Fig. 12.) 

The inventor of the collodion process was born at Stortford in 
1813 and in early life became a sculptor. He took up the Calotype 
process in 1847, it is said, for the purpose of making records of his 
work. We do not know just when he began experimenting with col- 
lodion but in 1850 his collodion process was so far advanced that he 
described it to a few friends, from whom he received some assistance, 
and the following year the details of the process were published in 


22 PHOTOGRAPHY 


The Chemist for March 1851. Archer appears not to have recog- 
nized the value of the process, for he did not patent it, but so com- 
plete and perfect was his process that it at once took the place of all 
other processes, remaining supreme in the field for almost thirty 
years, and is even to-day unsurpassed for certain branches of work. 


Fic. 12, Frederick Scott Archer. Drawing from an old print reproduced in 


J. Werge—The Evolution of Photography 


Archer was a fertile inventor and made several minor additions to 
photographic processes which we do not have the space to record, 
but was a poor business man, and upon his death in May 1857 in 
practically a state of poverty, the sum of 747 pounds was raised by 
subscription among friends, and shortly afterwards, Mrs. Archer 
having passed away, the Government granted the children a pension 
of fifty pounds a year as “their father was the discoverer of a sci- 
entific process of great value to the nation from which he had reaped 
little or no benefit.” | 


0 


THE DEVELOPMENT OF PHOTOGRAPHY 23 


The Collodion Process.—The following is an outline of the col- 
lodion process: ® 

I. Prepare pyroxyline by immersing cotton wool in equal parts of 
nitric and sulphuric acids for fifteen seconds, after which wash 
thoroughly in water. 

2. Dissolve the pyroxyline in a mixture of equal parts of sulphuric 
ether and absolute alcohol to obtain collodion. } 
3. Add some soluble iodide, preferably potassium, and also a little 

potassium bromide. 

4. Pour on a clean glass plate and allow to set. 

5. Take the coated plate into the darkroom and immerse in a bath 
of silver nitrate (thirty grains to the ounce of water) for a minute. 
Here a chemical change takes place resulting in the formation of a 
sensitive silver-bromo-iodide in the pores of the collodion. 

6. Place plate in holder and expose. 

7. lake plate back to darkroom and develop by pouring over it a 
solution of water, acetic acid, and pyrogallic acid. 

8. Fix by immersion in a bath of sodium thio-sulphate (“hypo’’). 

After the introduction of collodion, photography for the first time 
became really popular. Out of this newborn interest in the subject 
arose several institutions which were to stand until the present time 
and to exercise a favorable influence on the further developments of 
the science. The Royal Photographic Society was founded in 1853 
as the Photographic Society of London, and the following year the 
Societe Francaise de Photographie was organized at Paris. In 1854 
the well-known British Journal of Photography was established as a 
monthly, becoming a weekly in 1859, while the year previous had wit- 
nessed the birth of the Photographic News. 

Inconveniences of the Collodion Process.—While a notable ad- 
vance upon all previous processes, the collodion process was subject 
to several grave objections. It is absolutely necessary that the plates 
be exposed and developed as quickly as possible after their prepara- 
tion before the surface has had time to dry. For this reason the wet 
plate process, while well adapted to the studio, is not so suitable for 
landscape work, or for general amateur use. A heavy equipment 
had to be carried in the form of a tent, sensitizing bath, developing 
trays, fixing and developing solutions, and a plentiful supply of pure 


6 For a full description and formula see: Wet Collodion Photography, by C. 
W. Gamble; The Wet Collodion Process, Arthur Payne. 


24 PHOTOGRAPHY 


water. Some idea of the inconveniences of outdoor photography with 
the collodion process may be had from Fig. 13 and Fig. 14; the former 
shows the photographer “en route” with his outfit and the latter the 
outfit in use in the field. Sometimes the outfit was arranged to be car- 


Fic. 13. The Wet Collodion Process in the Field. 
(From an old manual) 


ried on a cart drawn by a donkey, an example of which is in the 
museum of the Royal Photographic Society of Great Britain, Fur- 
thermore if the exposure was a long one, as might easily be the case 
with interiors where exposures run to several hours, the surface of 
the plate dried and the picture was spoiled. Lastly in cold weather 
the sensitizing bath, solutions, water supply and plates would freeze, 
so that photography in winter, or in cold climates, was well nigh im- 
possible. 

Despite these obvious drawbacks, some of the work of this period 
ranks with the best that photography has produced. The work of 
Rejilander, the portraits of Solomon, Mrs. Cameron, and much of 
the famous work of H. P. Robinson were all done with collodion, and 
will ever remain as notable tributes to the enthusiasm and energy of 
these untiring workers. 


THE DEVELOPMENT OF PHOTOGRAPHY 25 


Modifications of the Collodion Process.—To overcome the defects 
of the collodion process John Spiller and William Crookes in 18547 
proposed the use of a deliquescent salt, such as magnesium nitrate, 
to keep the collodion moist and allow it to be kept several hours be- 
fore use. The same year George Shadbolt and Maxwell-Lyte advised 
the use of honey and grape sugar to prevent evaporation. The most 
successful method, however, was the collodio-albumen process devised 


Fic. 14. Portable Laboratory for Field Use with Wet Collodion 


by Taupenot in 1855.* In this process the plate after having been 
coated with iodized collodion in the usual manner was then flowed 
with albumen and allowed to dry, when it was immersed in a bath 
of silver nitrate, washed and dried. The plates so prepared were 
very slow, about six times slower than ordinary collodion, but would 
keep well and were rather extensively used by landscape workers. 

In 1855 Dr. Hill Norris of Birmingham described a process ® in 
which the plates were first washed in water and then immersed in 
pyrogallic acid, after which they were dried and kept until wanted. 
The following year he took out a patent for a collodio-gelatine process, 
the sensitive collodion plates being covered with a solution of gelatine 
in order to prevent its condensation on drying and to keep in a sensi- 
tive state. Dry plates so prepared were placed on the market and 
large numbers were sold between 1855-1866. 

Among other processes having as their object the production af dry 

* Philosophical Magazine. 


8 La Lumiere, Sept. 8, 1855. 
® Jour. Phot. Soc, of London (R. P. S.), May 1855. 


26 PHOTOGRAPHY 


plates we may mention the tannin process of Major Russel, intro- 
duced in 1861; the albumen-beer process of Capt. Abney, 1874; the 
resin process of the Abbe Desprats and the oxymel process of 
Llewelyn. 

The Introduction of Collodion Emulsion.—The term emulsion is 
applied to a liquid holding a large number of minute particles of a 
solid body in suspension. While Gaudin, and Dixon and Fry * had 
met with some success in the preparation of a workable collodion 
emulsion, it remained for Sayce and Bolton to work out the first 
satisfactory method for the preparation of a suitable collodion emul- 
sion for photographic purposes in 1864.11 These workers added 
nitrate of silver to a bromized collodion thus producing a sensitive 
bromo-silver collodion. Plates coated with this emulsion were flowed 
over with tannin and dried. Later improvements consisted in in- 
creasing the amount of silver and the addition of tannin directly to 
the emulsion. Many others added suggestions of importance, among 
whom ,may be mentioned Carey Lea, Col. Stuart Wortley, George 
Dawson, Thos. Sutton and W. J. Stillman. 

For several years after the introduction of collodion emulsion the 
excess silver salts were removed by washing the plates after coating. 
In 1871 Sutton suggested the use of a “ corrected ” emulsion in which 
the proportions of bromide and silver were so adjusted as to leave 
neither in excess, but because of the practical difficulties in determin- 
ing the proper proportions the method is not satisfactory. 

In 1874, Bolton showed that the emulsion might be washed before 
the plates were coated * and the following year Rev. Canon Beechey 
described ** a similar method using pyrogallic acid as a preservative. 
This method was perhaps the most reliable and uniform method of 
preparing collodion dry plates, and plates so prepared became an 
article of commerce. While not so rapid as ordinary wet collodion, 
the Beechey plates were sufficiently rapid for exterior work, requiring 
an exposure of from 30 to 60 seconds with a diaphragm equivalent 
to F/16. : 

The Introduction of Gelatino-Bromide Emulsion.—The first record 
of the application of gelatine to photography was the unsuccessful at- 

10 La Lumiere, Aug. 1853; Photo. News, 1861, p. 193. 

11 Brit. J. Phot., September 9, 1864. 

12 Brit. J. Phot., Jan. 16, 1874. 


18 Brit. J. Phot., Oct. 1, 1875. Harrison, History and Handbook of Photog- 
raphy, Appendix. 


THE DEVELOPMENT OF PHOTOGRAPHY 27 


tempt of Niepce de Saint Victor in 1847 as a vehicle for holding silver 
iodide on glass plates. 

In 1853, Gaudin gave a formula for what we would now term 
gelatino-iodide emulsion but his method was not practical. His exper- 
iments, however, led him to recognize the fact that bromide of silver 
is more sensitive in combination with gelatine than iodide of silver. 

The use of gelatine as a preservative of wet collodion by Norris in 
1856 we have already noticed under collodion emulsion. 

In 1868, W. H. Harrison * published the results of his experiments 
on the emulsification of silver bromide in gelatine but his method was 
of no practical value, the principal significance of his work being the 
use of an alkaline developer. 


Fic. 15. Dr. Richard Leach Maddox =. 


While Le Gray, Smith, Harrison and Sutton had either suggested 
the use of, or had experimented with gelatine, it is to Dr. R. L. Mad- 
dox (Fig. 15), an English amateur, that we owe the first really work- 


14 Brit. J. Phot., Jan. 17, 1868. 


28 PHOTOGRAPHY 


able method of preparing gelatino-bromide emulsions. His methoc 
was fully described in an article in the British Journal of Photography 
for September 8, 1871, under the following title: “An Experiment 
with Gelatino-Bromide.” The introduction of gelatine pointed the 
way to plates of a higher degree of sensitiveness than had been pos- 
sible with collodion, so that while the process of Dr. Maddox was not 
revolutionary and complete in itself as was that of Scott-Archer, it 
marks an epoch in the development of photography. | 

In the method described by Dr. Maddox, silver bromide was formed 
in the presence of gelatine, the emulsion containing an excess of silver 
and a small amount of aqua regia. Without further treatment the 
emulsion was coated on glass plates, dried, and exposed. Develop- 
ment was conducted with pyrogallic acid and intensification with pyro 
and silver nitrate followed. 

With our present knowledge it is not hard to see why Dr. Maddox 
did not meet with complete success. He does not seem to have real- 
ized the necessity for washing the emulsion so as to remove the excess 
silver salts, although this was regularly done with collodion emulsion 
processes. Consequently the presence of the excess salts of silver and 
the nitric acid from the aqua regia acted as a restrainer and made the 
plates very slow. 

It is noteworthy that Maddox had some idea of the ripening 
processes which have meant so much to the development of the 
gelatino-bromide process, as he tried to increase the sensitiveness of 
his plates by fuming with ammonia (a method that had been pre- 
viously applied to albumen paper) but without success. 


Very little attention was paid to the work of Maddox at the time, - 


but two years later Burgess advertised a gelatino-bromide emulsion 
in the English photographic journals.** The method employed by 
Mr. Burgess in the preparation of his emulsion was never published 
and did not prove to be a commercial success, but he was the first to 
show that excellent results could be obtained on gelatine, and that 
gelatino-bromide emulsion could be produced which was equal in 
sensitiveness to wet collodion. 

Improvements in the Gelatino-Bromide Process.—The same year 
that Burgess introduced his emulsion commercially, King and John- 
son described independently of each other two methods for removing 
the excess of silver salts from the emulsion. King’s method con- 


15 July 18, 1873. 


THE DEVELOPMENT OF PHOTOGRAPHY 29 


sisted in placing the emulsion in a container of vegetable parchment 
or bladder; the whole being immersed in a large vessel of water, un- 
der which circumstances the soluble salts pass outwards through the 
parchment into the water. Johnson’s process, described in the same 
issue of the British Journal of Photography,“ advised the use of an 
excess Of soluble bromide—a point of great importance—and plain 
washing of the shredded emulsion in running water to eliminate the 
excess salts. On account of its simplicity and effectiveness this 
method has been generally adopted. 

In November of the same year Richard Kennett, an amateur re- 
siding in London, took out a patent ‘* for a method which he discov- 
ered of preserving the emulsion and the following nionth announced 
his “ sensitive pellicle” which was nothing more than a dried, sensi- 
tive gelatino-bromide emulsion. The pellicle was quite successful and 
remained on the market for about ten years. 

In 1874 Bolton suggested that only a small part of the gelatine be 
used for preparing the emulsion, the rest being added afterwards—a 
procedure which later proved of great value when the effect of heat 
on the emulsion was discovered. The same year Stas observed that 
several forms of silver bromide are possible and that heating formed 
the most sensitive compound.1* The same year (1874) gelatine plates 
first appeared on the market, manufactured by the Liverpool Dry 
Plate Co. Bromide paper appeared at the same time. 

In 1878 Bennett showed that the sensitiveness of gelatino-bromide 
emulsion might be greatly increased by keeping the emulsion at a 
temperature of 90 degrees Fahr. for five to seven days.’ This added 
a great impetus to the development of gelatino-bromide emulsion and 
another firm of dry pais makers took the field—Messrs. Wratten 
and Wainright. 

The prolonged stewing of the emulsion at 90 degrees, however, was 
not only troublesome but led to trouble owing to the partial decom- 
position of the gelatine, so Mansfield announced in 1879 ”° that this 
long and troublesome process could be avoided by forming the bro- 
mide of silver in a weak solution of gelatine which was then boiled 
for ten to fifteen minutes, the remainder of the gelatine being added 

16 November 14, 1873. 

17B. P. No. 3782 of November 20, 1873. 

18 Annales de Chimie, Fifth Series, vol. III, p. 280. 

19 Brit. J. Phot., March 29, 1870. 


20 Brit. J. Phot., August 22, 1879. 
4 


30 PHOTOGRAPHY 


when the solution had cooled. Emulsification in a portion of the 
gelatine, the remainder being added after digestion, was a pines 
of the advice given by Bolton in 1874. Fy 

In May 1879 Captain Abney showed that a good emulsion fate 
be formed by precipitating silver bromide in glycerine, the precipitate 
after two or three washings with water being mixed with the proper 
amount of gelatine to form the emulsion. The object of this method 
was to save the trouble of washing the emulsion in order to eliminate 
the excess silver salts as required in the usual process. 

In 1877 Johnson described the use of an aqueous solution of am- 
monia for the ripening of gelatino-bromide emulsion.7* Not much at- 


tention seems to have been paid to this communication, however, and 


it was not until Monkhoven in 1879 ** suggested that the increased 
sensitivity of the emulsion produced by prolonged heating might be 
due to a change in the molecular state of the silver bromide along the 
lines of the work of Stas and showed that silver bromide might be 
changed from the ordinary to the most sensitive green state by treat- 
ment with ammonia that much interest was taken in the subject. The 
following year Eder investigated the matter very thoroughly and per- 
fected a process using ammoniacal silver oxide and later discovered 
the advantageous influence of ammonia and ammonium carbonate on 
the ripening of gelatino-bromide emulsion in the cold.2* The follow- 


ing year Abney showed the advantage to be gained from the use of a 


small amount of iodide in gelatine emulsions. The addition of iodide 
at first reduced the speed of the emulsion to a certain extent, gave 
clearer negatives having greater density. A small percentage of 
iodide is used in nearly all modern plates. 

In the meantime, the spread of gelatino-bromide emulsion had been 
exceedingly rapid and by 1882 gelatine emulsion had almost com- 
pletely displaced collodion, except for some few specialized purposes. 

Development of Printing Processes with Silver Salts.—The de- 
velopment of positive printing processes begins with Fox-Talbot’s 
Calotype process in 1841, although it was not until after the inven- 
tion of the collodion process that much progress was made in this 
line. . 

For printing from his early negatives, Fox-Talbot employed what 

21 Brit. J. Phot. Almanac, 1877, p. 95. 


22 Brit. J. Phot., October 17, 1879. 
23 Sitzungsber. Akad. Wiss. Wien, 1880, 81, 679. 


THE DEVELOPMENT OF PHOTOGRAPHY 31 


we to-day term the salted paper process. Paper of suitable surface 
and texture is immersed in a weak solution of salt, after which it is 
dried, and in this state it may be kept indefinitely. Just before use, 
it is sensitized in silver nitrate and dried, after which it is exposed to 
daylight under the negative to be reproduced. When the image is 
sufficiently dark, the print is removed and toned in a solution of gold 
chloride which is followed by fixing in a bath of “hypo.” Talbot’s 
early prints, however, were not toned, as gold toning does not seem to 
date back further than 1849. We will have more to say regarding 
plain salted paper later on as it is to some extent in use at the present 
time. ; 

Le Gray appears to have been the first to suggest coating the paper 
with albumen before sensitizing in order to obtain a higher gloss, al- 
though Fox-Talbot is often credited with the same. Albumen paper 
was quite popular and practically the only paper used from 1860- 
1885. ; 

Palmer and Smith as early as 1866 showed how a paper coated *4 
with an emulsion of gelatino-chloride of silver might be used for 
positive printing. Further details were given by Dr. Eder, Capt. 
Abney and W. T. Wilkinson in 1881 and in 1885-867° a valuable 
series of papers on the subject by Ashman and Offord appeared in 
the Photographic News. Gelatino-chloride paper appeared on the 
market shortly afterward and was quite popular both in America and 
in England, where it was used long after it had practically disappeared 
in this country. 

Collodio-chloride paper appears to have originated with G. Wharton 
Simpson in 1864.2 Under the name of Aristo Platino it for a long 
time remained the standard printing medium in America and was only 
displaced by the advent of developing-out papers. 

Gelatino-citro-chloride paper was introduced by Abney in 1881 and 
placed upon the market by Liesegang of Dusseldorf in 1886 as Aristo. 

The above papers are all members of the class known as P. O. P. 
(printing-out papers) ; that is they produce a visible image upon ex- 
posure. The now popular developing papers appear to have had 
their prototype in a process used by Blanquart-Evrard in 1851. In 

24 Phot. News, 1866, pp. 24, 36. 

25 Eder, Phot. News, 1881. Abney, Phot. News, 1881. Wilkinson, Brit. J. 


Phot., 1881, pp. 140, 168. 
26 Photographic Yearbook, 1865, p. 63. 


32 PHOTOGRAPHY 


his process, however, silver iodide and not the bromide or chloride 
was used.,?” 

The first paper for positive printing coated with a silver-bromide 
emulsion was introduced by the Liverpool Dry Plate Company in 
1874, but does not seem to have attracted much attention at the time. 
In 1880 Morgan and Kidd established a factory at Richmond and 
gradually Ilford, Barnet, Eastman in America and other manufac- 
turers followed until now there are numberless brands of bromide 
papers of various varieties and surfaces. 

In 1893 Velox, the first of the “ gaslight” papers, was introduced 
by the Nepera Chemical Company from the formula of Dr. Leo 
Raekeland. This is a chloride emulsion developing-out paper without 
free silver, which is very much slower than bromide paper and can 
be handled in a brighter light. Since the advent of Velox many other 
similar brands have appeared both in this country and England, and 
indeed all over the world, and are now by far ae most widely used 
papers for positive printing. 

Platinum Printing Processes.—In 1832 Herschel discovered that 
light had a reducing action on platinum compounds, especially in the 
presence of an organic salt such as ferrous oxalate. Hunt in 1854 
tried to turn this to account by coating paper with ferric oxalate and 
platinic chloride, but he failed to realize an essential point, the two 
salts must be in solution before the reaction can take place. It is to 
William Willis Jr. that we owe the platinum process in its present 
form. He took out his first patent in 1873, a second in 1878 and the 
last in 1880. Under the last patent, paper is coated with a mixture of 
potassium chloroplatinite and ferric oxalate. This is exposed under 
the negative until the image is sufficiently printed when it is removed 
and placed in a solution of potassium oxalate, in which the reduced 


iron salt is soluble, and as it is dissolved by the oxalate it attacks the — 


platinum compound and reduces it to the metallic state. After im- 
mersion in several baths of hydrochloric acid to remove the iron salts 
which remain, the print is washed and dried. 

Printing Processes with Bichromated Colloids——In 1839 Mungo 
Ponton discovered that paper sensitized in ammonium bichromate 
changes from yellow to brown on exposure to light and in 1855 
Poitevin took out a patent for a process involving chromatized gela- 
tine to which a pigment had been added. John Pouncy exhibited 


27 Phot. News, 1856, p. 63. 


“5 
“v 
a 
he > a) * 
o Le ee ae a ae 


THE DEVELOPMENT OF PHOTOGRAPHY 33 


before the Photographic Society of London in 1858 a number of 
carbon prints but received little but ridicule for his efforts, which 
were faulty, as was to be expected for a first attempt. Gum bichro- 
mate, so much in favor with certain pictorialists, may be said to date 
from Pouncy’s work but it was not until brought to the front by 
Demachy and others at a later date that it became really popular. 

Carbon printing for the first time became really practical in 1864 
when J. W. Swan introduced carbon tissue and the final step was 
made ten years later when Sawyer invented the flexible support, after 
which carbon quickly took its place as one of the really great print- 
ing processes. 

The modern oil process is a development of the collotype process 
patented by Poitevin in 1855, the principal difference being in the 
local application of the ink by brushes rather than a roller. The mod- 
ern method is due largely to Mr. G. E. H. Rawlings. 

Bromoil, a method of converting bromides into a condition suitable 
for pigmenting, was developed by C. Welborne Piper in 1907.?8 

Carbro, a method of making carbon prints from bromides, is a de- 
velopment of the Ozobrome process as introduced in 1905 by Thomas 
Manly. We will have more to say regarding the history of the vari- 
ous printing processes in later chapters dealing with the subject of 
printing. 

Conclusion.—With this our history of the development of photog- 
raphy must be brought to a close. Many are the names and processes 
which we have been compelled to barely mention and not a few have 
been omitted altogether, while all have been treated in outline only, 

28 Mr. E. J. Wall has called my attention to the fact that the first suggestion 


regarding the bromoil process was made by him in the Photographic News for 
1907, p. 209. The passage to which Mr. Wall refers is as follows: 


“Suppose we enlarge direct on to a bromide paper and develop with a 
non-tanning developer, such as ferrous oxalate, we should obtain an image 
in the ordinary way in metallic silver. If this image were treated with a 
bichromate, the gelatine should be rendered insoluble in proportion to the 
amount of silver present, just as though exposed to light. One would then 
only have to dissolve out the unaltered bromide and the metallic silver with 
hypo and ferricyanide to obtain an image in insoluble gelatine, to which the 
ink or pigment should adhere as in the original oil process.” 


While to Wall undoubtedly belongs the credit of having first suggested the 
rationale of the bromoil process, to Welborne Piper belongs the credit of having 
worked out the details of the same, and having brought. it before the world in a 
practical form. 


34 PHOTOGRAPHY 


so that only a general idea of their essentials has been gained. It is 
hoped, however, that this short account has been of sufficient interest 
to encourage the student to follow up the subject by outside reading 
in larger and more comprehensive works and to assist in this worthy 
end a short list of the leading historical works on photography is 
appended. 


GENERAL REFERENCE WORKS 


BrotHErs—A Manual of Photography. London, 1899. ; 
Brown—Who Discovered Photography? Photo-Miniature, No. 60. 1903. 
CoLsEN—Memories des Createurs des Photographie. Paris. 
Eper—Geschichte der Photographie. Halle a/S. 

Eper—Johann Heinrich Schulze. 1917. 

FouguEe—Sur la Invention de la Photographie. Paris, 1867. 
Harrison—The History of Photography. London and New York, 1885. 
LicutTFIELD—Tom Wedgwood, The First Photographer. London. 
PotonNIEE—Historie de la Decouverte de la Photographie. Paris, 1925. 
SCHEINDEL—Geschichte der Photographie. 

TISSANDIER—History and Handbook of Photography. London, 74 
WercE—The Evolution of Photography. 


—e- 


CHAPTER II 
THE CAMERA AND DARKROOM 


I. THE CAMERA 


The Box Camera.—The simplest and the cheapest possible camera 
which you can purchase is one of the box form. Such cameras are 
cheap because they are made in large quantities by machinery and 
because they do not have the capabilities and adjustments of more 
expensive models. Naturally they are more limited in their scope 
and cannot be used for a wide range of work. However, since they 
are simple and easily operated they form an excellent camera for 
the beginner, who has not yet become familiar with the various ad- 
justments which render the more expensive instrument capable of 
handling a wide range of subjects more efficiently. 

Box cameras are supplied in several sizes from 15¢x2™% to 
3% x5 inches and in both roll film, pack and plate models, 
although the last named have practically disappeared from the com- 
mercial market. On account of the greater bulk of the camera the 
smaller sizes are most popular and since the roll film or film pack 
models are lighter and more convenient to use they have practically 
superseded plate cameras of this type. Box cameras are generally. 
fitted with cheap single achromatic lenses which cannot be used at a. 
larger aperture than F/16. In bright light, from 8 to 3 o’clock, snap- 
shots may be made if the subject is open and not in shadow. Under 
other conditions the camera must be placed on a firm support and a 
time exposure given. Since the positions of both lens and film are 
fixed it is impossible to focus and the lens is so placed that all objects 
from infinity to within 10 to 15 feet of the camera are defined with 
satisfactory, if not critical sharpness. This avoids one of the dif- 
ficulties of the beginner and hence such cameras, under the proper 
conditions of light, give good results with the minimum of trouble 
and skill on the part of the user. 

The Miniature Camera.—The miniature camera, or V.P. camera 
as sometimes termed, ranges in size from 4%4 x6 cm. (1.77 x 2.36 
inches) to 64% x9 cm. (2.56x3.5 inches), is more expensive, and 

35 


36 PHOTOGRAPHY 


is designed to be fitted for a wide range of serious work with the 


minimum of inconvenience to the owner when not in use. The par- 
ticular feature of these cameras is their portability. They are small 
and light, so that they may be carried in an ordinary pocket without 
annoyance and brought into use quickly and with the minimum of 
effort when desired for use. At the same time such cameras are 
capable of really serious work, when they are handled with skill, since 
when fitted with good lenses the small negatives enlarge readily to 
medium sizes. A typical example of an instrument of this type is 
shown in Fig, 16. 
When purchasing a camera of this kind it is well to remember that 
although they are rather expensive it is well to get the best and espe- 
cially to secure a good lens since good sharp definition will be required 
for subsequent enlargement, while a large aperture will enable snap- 
shots to be made when otherwise impossible. Another important 
thing to examine is the soundness of construction. In attempting to 


Fic. 16. Typical Miniature Camera for Plates and for Roll Film 


make the camera small and light many manufacturers have rather 
lost sight of stability and their instruments are flimsy and easily de- 
ranged. While a certain sacrifice in stability is necessary in order to 
prevent undue weight and size it is desirable that the instrument be 
sufficiently strong to withstand long-continued usage. 

Many cameras of this type are rather overloaded with adjustments 
and movements, which are useful at certain times but are more often 
simply a hindrance to fast work. In the opinion of the writer the 
following are the most important features of a miniature camera: 


=~") 2 


] 
| 
, 
q 
a 


THE CAMERA AND DARKROOM 37 


I. Body of aluminum or better Duraluminum. 

2. A platform so that the lens is covered when the camera is folded. 

3. When opened the front should lock with the lens in focus for 
objects at a medium distance, say 15 to 30 feet. 

4. Further focussing should be provided for with either a lever or 
pinion, conveniently located. 

5. The focussing scale, shutter speed and diaphragm scales should 
all be visible from the top of the camera so that any adjustments may 
be made while the subject is being followed in the finder. In the 
case of cameras designed for use with direct view finders the shutter 
and diaphragm scales should be visible from the viewpoint of the 
eye when following the subject in the finder. | 

6. The finder should be placed as close as possible to the lens in 
order that the correspondence between the two may be as perfect as 
possible. 

7. The lens should be a high-grade anastigmat, with a large aper- 
ture, as F/6.3 or F/4.5, in a shutter with a wide range of speeds from 
one second to 1/200. 

The advantages as regards convenience certainly lie with the min- 
lature camera using roll film but many of the disadvantages of plates 
are removed when small sizes are used. Thus weight becomes neg- 
ligible, and the only remaining difficulties are those of loading and 
unloading plate-holders, while the advantages of focussing and selec- 
tion of particular plates for different purposes are valuable to the 
serious worker. At the same time when facilities are lacking for 
the use of plates, film packs may be used in an adapter which may be 
loaded or unloaded in daylight and focussing done just as with plates. 
Film packs thus offer the same advantages as both films and plates; 
the only objection at present is that film packs are only made in one 
speed and brand of color sensitiveness. 

Folding Hand Cameras.—Folding hand cameras are made in sizes 
from 24x 3% to 4x5 in both film and plate models. There 1s 
without doubt a greater demand for this class of instrument than any 
other—a fact which is evident from the wide range of models pro- 
vided by the various manufacturers. For one thing, the contact 
prints are sufficiently large to satisfy the requirements of the average 
amateur, while the instrument itself, although less portable than the 
miniature camera, is easily slipped into the coat pocket or slung over 
the shoulder by means of a leather strap. 


38 PHOTOGRAPHY 


Roll film models of this class call for the barest mention since they 
are the cameras in general use by the larger body of amateurs. They 
are ideally fitted to the needs of most amateurs for whom the camera 
is only a method of keeping a record of their happy experiences on 
trips and during vacation. For serious photographic work of a gen- 
eral nature they are not so well adapted since they are not provided 
with means of focussing on the ground-glass, and consequently can- 
not be used for copying and work of a similar nature, while there is 
some uncertainty in general view or portrait work owing to the fact 
that it is hard to gain a proper idea of the subject from the small 


Fic. 17. Hand Cameras for Plates and for Roll Film 


image in the finder, which ‘moreover is seldom in accurate register 
with the lens. For this reason many prefer to purchase one of the 
light plate cameras and use film-pack whenever the advantages of 
lightness and daylight loading are important. 

The more expensive plate cameras of this class are exceedingly ver- 
satile instruments and are capable of doing almost anything that the 
average photographer is likely to demand. They are fitted with re- 
versible backs so that pictures may be made either vertically or hori- 
zontally without turning the camera on its side and many of them 
have a long bellows which enables them to be used for copying and 
photographing small objects. In addition, the long extension permits 


THE CAMERA AND DARKROOM 39 


the use of long-focus lenses, which for portraiture and certain kinds 
of landscape work are very desirable. 

Some of these cameras are also fitted with a swing back which en- 
ables the plate to be kept in a vertical position while the bed of the 
camera is tilted upward in order to include the whole of a tall object 
on the plate. Various other features are supplied on the different in- 
struments such as rising and falling front, wide angle bed for using 
wide angle lenses, and sometimes sliding fronts are fitted. Since these 
cameras are always fitted with a finder and focussing scale they may 
be either held in the hand or placed upon a tripod. They are thus 
suitable for both the most exacting work and at the same time may 
be used as a hand camera whenever desired. ‘There is little ques- 
tion that this is the most efficient instrument for really serious photo- 
graphic work which the amateur can buy. Typical examples of hand 
cameras for both plates and film are shown in Fig. 17. 

The Professional Camera.—The view and studio cameras in gen- 
eral use by the professional photographer do not differ greatly from 
the folding plate camera which we have just described. They are 
usually more substantial and consequently more bulky, while they are 
of larger size—the ordinary sizes being 5x7, 7x11, 8x10 and 
11x14 inches. In addition to greater stability, the professional 
cameras have greater bellows extension, and the various adjustments 
have greater latitude than in the folding compact hand camera already 
referred to. The lens board is also larger so that long focus lenses 
of large aperture may be accommodated. In addition there is pro- 
vision for focussing from either the back or the front—a valuable 
feature when wide angle lenses are in use, since in this case focussing 
may be done from behind and there is no danger of a part of the 
camera bed appearing in the picture. Some cameras of this type 
known as banquet cameras, made for such work as their name indi- 
cates, have an arrangement by which the lens may be tilted down- 
ward while keeping the plate vertical, so that large groups may be 
photographed from above with the minimum of distortion (Fig. 18). 

The studio camera is in general similar to the view camera except 
that it is much heavier and larger, the rising front is dispensed with 
as is also front focussing. The lens board is larger so that the large 
bulky portrait or anastigmat lenses of long focus and large aperture 
may be readily accommodated. 


40 PHOTOGRAPHY 


The. Reflex Camera.—The principle of the reflex camera requires 
a word of explanation since it is radically different from any of the 
cameras which we have already described. Fig. 19 shows a typical 
reflex in cross section. The rays of light from the object pass through 


Fic. 18. Professional View Camera 


the lens and are reflected by the mirror to the focussing screen at 
the top of the camera, where the image is of course in its normal 
unreversed position, i.e. right side up. Behind the mirror is the focal 
plane shutter and behind this the sensitive plate. The focal plane 
shutter consists of a long opaque curtain with apertures of varying 
lengths, any one of which may be made to pass across the front of 
the plate at a high speed. When the image has been focussed on the 
ground-glass and the exposure lever is depressed the mirror swings 
up out of the way and forms a light-tight joint with the focussing 
screen. As soon as the mirror reaches this position it automatically 
operates the shutter. Thus two distinct operations are performed in 
the interval between the action of the exposure lever and the actual 
exposure. First the mirror is released and swings up, and then an 
aperture in the curtain of the focal plane shutter passes over the 
plate and makes the exposure. However, in a well-made reflex the 
mirror and shutter are so well coordinated that the time interval is 
not more than 1/10 to 1/5 of a second. 

The reflex camera offers several distinct advantages possessed by 
no other camera, which renders it well worth its cost, which is neces- 
sarily rather high owing to the care needed in manufacturing and 
properly adjusting the intricate mechanism. The image can be seen 


THE CAMERA AND DARKROOM 41 


in full size, right side up on the focussing screen until just before the 
exposure is made. Thus the reflex is superior to the folding film 
camera in that it is possible to focus accurately on the ground-glass 
and not have to depend upon focussing scales. It is superior to the 


Fic. 19. Principle of the Reflex Camera 


.ground-glass focussing plate camera in that the image is right side up 
so that composition and placement of the subject is simpler and also 
in the fact that the exposure can be made immediately without the 
operations of closing the lens, inserting the plate-holder, withdrawing 
the slide, etc. Furthermore, very rapid exposures are possible since 
the ordinary focal plane shutter works up to a maximum speed of 
1/1000 second. 

Aside from its expense, the principal objection to the reflex is its 

bulk and weight. There is no doubt that where portability is an im- 
portant factor the average reflex is rather out of question. The 


42 PHOTOGRAPHY 


31%4xA4% instrument weighs from four to five pounds and occupies 


a space of approximately 5 x 5 x 6 inches, while the 4x 5 size is cor- © 


respondingly larger. The first mentioned size is the more popular of 
the two. For those who demand portability and yet desire reflex ad- 
vantages the 214 x 3% size may be recommended, while the 5 x 7 size 
is practically obsolete except among professional workers. 

To overcome the bulk of the box form reflex many manufacturers, 
especially in foreign countries, have placed folding models on the 
market. These are much more costly, and are neither as substantial 
in construction nor do they possess the usual bellow extension, extent 
of rising and falling front, etc., so that at present the author is of the 
opinion that the box form type is still the best. While especially 
suitable for photographing objects in rapid motion, the reflex is by no 
means limited to work of this class. Indeed for all ordinary work it 
is the most certain and convenient instrument to use. It is of course 
not suited to architectural work when a swing-back is required and 
in most cases cannot be well used for copying, but for all general 
work in the field or at home the reflex is ideally adapted. 

The Principal Adjustments of Cameras.—The principal adjust- 
ments of cameras are the rising and falling front, the vertical swing 


or swing-back as it is commonly termed, the horizontal or side swing, - 


and the reversible back. 

The rising and falling front is an arrangement for raising or 
lowering the position of the lens in order to increase or decrease the 
amount of foreground included. While at times necessary in all 
kinds of work, it is particularly valuable in architectural work where 
it 1s necessary to include the whole of a tall building. The amount 
which the lens may be raised is usually expressed as a fraction of the 
greatest length of the plate. Thus if the rising front on a 4x5 
camera allows the lens to be raised one inch above its normal central 
position the degree of rise is said to be 1/5. The amount which the 
lens can be raised varies in different makes of cameras but is always 
greater in view cameras than in the more compact hand and stand 
cameras. 

Wide limits of rising front are sometimes required in exacting 
cases and at any rate it is well to secure a camera allowing the maxi- 
mum rise and fall for a camera in its class as the reserve rise will at 
times help one out of difficulty. To secure the full advantages of a 
camera having extreme rise, a well-corrected lens with a reserve 


—— = — 


—— ae 


THE CAMERA AND DARKROOM 43 


covering power is required, since when the lens is raised above its 
normal position it is the margins of the field rather than the center 
that are used and consequently the greater the demand for good cor- 
rection, since the definition of a lens is never so good near the margins 
as at the center. Reserve covering power is needed in order that the 
plate may be completely covered when the lens is fully raised. 

While many film cameras are provided with a rising and falling 
front its utility is in this case somewhat doubtful, since the finder 
cannot be relied upon to show just what is included when the lens 
is not in its normal position. Several makes of cameras, however, are 
fitted with self-adjusting finders which more or less accurately indi- 
cate the exact limits of the picture when the lens is raised above its 
normal position. . 

The Swing-Back.—The swing-back is an adjustment for swinging 
the back of the camera at an angle to the bed so that the plate may 
be kept in a vertical position, when the camera is pointed upwards in 
order to include a lofty subject on the plate. In a of Fig. 20 the 
camera is supposed to be absolutely level so that the parallel lines of 
the subject 4 and B are represented by parallel lines A’ and B’ in the 
image formed by the lens L. In this case there is no distortion. 
However, when the camera is tilted upwards as in b of Fig. 20 the 
sensitive plate is no longer parallel with the subject AB and conse- 
quently the parallel lines A and B of the subject are represented as 


Fic. 20. Principle of the Swing Back. 
Use of the swing back for securing greater depth of focus 


converging lines in the image. However, if the camera is fitted with 
a swing-back, the plate can be brought to a vertical position by prop- 
erly adjusting the back and distortion will be avoided although the 


44 PHOTOGRAPHY 


bed of the camera be tilted upwards. However, as will be observed 
from c of Fig. 20, the axis of the lens is no longer at right angles to 
the sensitive plate, but crosses it obliquely, so that the use of a small 
diaphragm is necessary to obtain sharp focus over the entire area. 
The size of the diaphragm required depends upon circumstances and 
can only be determined by examination of the ground-glass image. 
The other use of the swing-back is in focussing different planes at 
varying distances from the camera sharply, without using small dia- 
phragms. Suppose we are required to photograph the side of a hill 
on which A and B represent objects of interest which it is necessary to 


focus sharply without the use of small diaphragms. With the plate in © 
the horizontal position CD it is evident that the distance BD is greater 


than AC and that only one of these objects can be sharply focussed 
unless a small diaphragm is used. However, if the swing-back is ad- 
justed so that the sensitive plate occupies the position DE, the distance 
of the plate from 4 is increased while BD remains the same. By 
watching the focussing screen while making these adjustments it is 
easy to bring about some compromise which will allow a larger stop 


to be used than would otherwise be possible. In every case in which | 


the axis of the lens is not at right angles to the plate a certain amount 
of distortion results, so that this movement cannot be used for certain 


kinds of work, but on landscapes, portraits, etc., the small amount of — 


distortion may pass unnoticed. Indeed in some cases it is a positive 
advantage since it emphasizes the nearer objects. At any rate the 
worker must determine for each particular case, by examination of 
the image on the ground-glass, whether the distortion is objectional 
or not. | 

The Reversible Back.—The reversible back allows the back of the 
camera to be reversed so that the picture can be made either hori- 
zontally or vertically without turning the camera on its side. This is 


a very convenient feature and is found on all of the more expensive © 


plate cameras except those which must be made extremely compact. 


It cannot be had on any roll film camera, all of which must be re- 


versed when a horizontal picture is required. Some cameras are fitted 
with what is termed revolving backs instead of reversible backs. 
These serve the same purpose but, as their name indicates, they are 
revolved from one position to the other without being detached from 
the camera. 

Other Movements.—Some cameras are also fitted with side swings 


te ae es see a eos, | 7 


THE CAMERA AND DARKROOM 45 


which allow the plate to be adjusted with reference to horizontal ob- 
jects. While at times valuable, the horizontal or side swing is not 
nearly so important as the vertical swing or swing-back, and for that 
reason it too is found only on the professional view camera and the 
more expensive hand and stand cameras. 

The sliding front is an adjustment fitted to only a few cameras and 
these are generally view cameras for professional use. When two 
pictures are made on the same plate, the lens may be moved to each 
side in order that the center of the field may be used. 

There are many other adjustments fitted to various makes of 
cameras which are not sufficiently universal in application to require 
attention. 


II. THe DAarKRooM 


The Size of the Darkroom.—The important part played by the 
darkroom in the quality and volume of the work produced is not as 
fully realized as it should be. As a consequence, we have many dark- 
rooms which are mere makeshifts, which are ill arranged and result 
in serious loss of valuable time and materials, and in some cases in- 
jurious to the health of the photographer and to the sensitive ma- 
terials he uses. It is well worth while to pay particular attention to 
making the darkroom an orderly, well-arranged place, which is both 
healthful and pleasing, a place which one will not object to living in. 

The size of the darkroom is in most cases determined by the cir- 
cumstances attending to its location. The proper size is determined 
by the character as well as the volume of work carried on. For the 
average amateur there is no particular advantage in a room larger 
than ten by twelve feet, while about six by six feet may be regarded 
as the minimum. The advantages of a workroom about eight by ten 
or ten by twelve feet are: the greater ease in heating and ventilating, 
and less danger from stray light from openings in the walls or from 
around the door or window. A room with these dimensions allows 
an enlarging lantern to be installed and provides room for separate 
benches and sinks for different operations, as plate changing, plate 
developing, printing, washing, etc. 

For commercial work no definite size can be stated, as this will de- 
pend upon the class of work and upon the volume of business. In any 
case, room should be provided to allow sufficient space for each op- 
eration so that there may be a separate and distinct place for each 


5 


46 PHOTOGRAFPAY 


operation and for the materials and equipment required for this par- 
ticular operation. When this is done things are not so often mis- 
placed, broken, overlooked or destroyed. For a large business it is 
an advantage to divide the workroom into several smaller workrooms 
each of which is equipped and used for one particular purpose and 
no other. Thus we may have one moderate-sized room for plate 
changing and development, another for printing, and still another for 
the storage of chemicals and for preparing solutions. Of these three 


rooms in most cases the printing room requires to be the largest, and © 


the room for developing next in size, while the chemical storage room 
may be comparatively small since it is not in constant use. If only 
a small amount of enlarging is done the apparatus may be installed in 
the printing room, but if enlarging forms an important part of the 
business it is well to provide a separate place for this purpose. 
Ventilation.—A matter of particular importance, to which little or 
no attention is generally paid, is proper ventilation. In the opinion 


of the writer it is undesirable to have as a darkroom one which is ~ 


permanently dark. If conditions will permit, it is far better to have a 
room which includes at least one window, or more if the room is 
large, which is provided with a tight-fitting, light-proof blind which 
may be quickly opened to admit air and light and as quickly closed for 
work. While this will be of considerable service in ventilating the 
darkroom, and may be sufficient for the amateur who only works for 
a short period of time, something more is required in large establish- 
ments where the room is used throughout the day. Here it is neces- 
sary to provide light-proof air vents in order to allow the entry of 
fresh air and if the exit vents are fitted with suction fans so much 
the better. We illustrate in Fig. 21 a plan for the ventilation of the 
workroom which will be found quite satisfactory. A ten-inch pro- 
peller fan will handle about 300 cubic feet of air per minute and is 
large enough for a room containing about 4000 cubic feet of air. 
While this may seem elaborate and unnecessary expense, it can be 
proved that the gain in general efficiency more than compensates for 
the initial cost while the good will of the employe cannot be estimated 
in dollars and cents. 

Arrangement.—The arrangement of the darkroom is a matter de- 
serving particular attention. In laying out the floor space the aim 
should be to allow plenty of space to enable an operation to be car- 
ried on quickly and efficiently without hindrance to other work which 


THE CAMERA AND DARKROOM 47 


may be going on at the same time. To do this there should be a 
separate place for the materials and apparatus for each operation and 
this should be convenient to the place where the work is carried on 
so that articles required may be readily accessible. Forethought along 


SUGGESTED LOCATION OF INLET & EXHAUST. 
T SHOULD BE ON OUT SHOE Watt 
TF POSSIBLE 


lion Lock VENT. LOCATE JN WALL 
Jo Zewe IN AUR FROM SurTA8LE PLACE 


FRUNT INSIDE FLAT 
INDIAN RED 


Fic. 21 Ventilation of the Darkroom. 
(Courtesy of Eastman Kodak Company) 


these lines and careful planning of the workroom with a view to the 
requirements will save much time and labor later. 

Two very suitable arrangements for the amateur are shown in 
Fig. 22, one for a room and the other for a closet which is to be con- 
verted into a workroom. It will be observed that in both cases the 
space for loading plates and developing is placed behind the door 
where there is less danger from stray light. The space for loading 
plate holders or development is marked A in both floor plans, while 
B represents the sink in which the fixing bath and washing tanks are 
kept. The enlarging lantern may be conveniently placed in either at 
C and a separate bench for the printing machine and cabinets for 
papers and plates may be placed along one of the free sides of the 
room. 

The Water Supply (Sinks).—As a large amount of water is re- 
quired for most photographic operations, the water supply is an im- 


48 | PHOTOGRAPHY 


portant item in the location of the darkroom, which should be lo- 
cated, if possible, so that water may be easily installed. For while 
running water is not an essential, at least for the amateur who works 
at intervals, though it may be regarded as an absolute necessity for the 
professional, it is a decided convenience and adds much to the pleasure 


vi 
ee) 
i) 
Fe 
Rad 
| 
«3 
a) 


Fic. 22. Floor Plan of Darkroom for Amateur Use 


of the work. Owing to the location of the darkroom it may be im- 
possible to install running water either because the mains are not 
available or the cost is prohibitive. In such cases the amateur will 
find a very good substitute in a large water cooler of about five gallon 
capacity. This should be fitted over the sink in such a position that 
the tap is conveniently located for the drawing of water for the dilu- 
tion of solutions and rinsing of plates after development. A similar 
container may be placed below the sink. Operations which require 
a large amount of water, as the washing of plates or prints, will then 
be carried out in another room where running water is available. 
Opinions vary regarding the proper size and the construction of the 
sink. In the opinion of the writer it is a mistake to have a sink 
smaller than 18 by 36 inches. A sink of this size is just sufficient to 
carry a cold and a hot water tap together with the negative fixing 
tank, which should always be kept in the sink in order that there may 
be no danger from hypo infection. Larger sinks offer advantages in 
that they may contain in addition the negative washing box and the 
print washer or other similar apparatus. On the other hand they 
quite frequently occupy valuable room and are otherwise objectionable 


because they keep the room damp and unpleasant and cause metal — 


goods, as the enlarging lantern, to rust. In arranging for the sink 
the worker must be guided by his own requirements and by the space 
available. The sink itself may be of wood coated with a waterproof 


THE CAMERA AND DARKROOM 49 


paint, such as Probus; of cement, of enameled steel, or of lead. Steel . 
enameled sinks are perhaps the most satisfactory and are really the 
cheapest in small sizes but are obtainable only on special order in 
very large sizes and are also very expensive. Large sinks are there- 
fore generally made of eithér cement or wood coated with a .water- 
proof paint. Taking into consideration the labor involved in con- 
structing the same, the wood sink is the cheaper and is perfectly sat- 
isfactory, provided it is kept well coated with a water-, acid- and 
alkali-proof paint. In the laboratories of the Division of Photog- 
raphy at The Pennsylvania State College, the writer for several years 
had two wooden sinks, covered with an alkali- and acid-proof paint, 
in almost constant use and they have proved perfectly satisfactory. 
The only precaution to be taken is to renew the coating of paint once 
or twice a year. The majority of sinks, however, are made of con- 
crete, which is resistant to all acids and alkalies of such strength as 
are used in ordinary photographic practice. The following directions 
for the manufacture of a large concrete sink were given at a meeting 
of the Photographers’ Association of America some twelve or thir- 
teen years ago. A framework of half-inch boards is first built on 
the support where the sink is to be placed, and on this a thick layer 
of cement and sand in the proportion of cement two parts and sand 
three parts is laid, about an inch thick. While this is setting, an in- 
ner framework of half-inch boards, about two inches shorter than 
the outer one and without any bottom, is prepared and when the bot- 
tom layer of cement is set, this inner framework is rested upon it, 
and the tops of the inner and outer framework are kept steady at a 
distance of about an inch apart by two strips of wood attached at 
distances at the top. This forms a mould between the two frame- 
works and the bottom layer of cement, and into this mould more 
cement mixture is poured and allowed to set. The waste pipes 
should be put in before the cement sets and placed a little below the 
surface to allow for the shrinkage which occurs upon drying. To 
strengthen the sink large nails, or pieces of iron or steel, may be im- 
bedded in the cement and if thoroughly covered they will not rust. 
When the cement has become thoroughly hard the forms may be re- 
moved and work begun immediately. If the somewhat rough surface 
is objected to for any reason the cement may be coated with any of 
the compounds used for finishing cement surfaces and will then be 
perfectly smooth and resistant. 


50 PHOTOGRAPHY 


In fitting taps over the sink care should be taken that they are not 
so low that large graduates or containers cannot be placed under them. 
Placing the taps too high is also to be avoided owing to the trouble 
from splashing. A height of about fifteen inches from the bottom of 
the sink is a fair distance. 

The Illumination of the Darkroom.—There are two systems of 
darkroom illumination, direct and indirect. With the former we are 


already acquainted, but more will be said on this subject shortly. In-_ 


direct light, while having been adopted by some large commercial 
houses, has been neglected by the amateur and even by the average 
professional. The advantages of indirect light are many, as will soon 
be observed by one who installs it, and the amount of light which may 
be present in the darkroom without danger of fog on even the most 
sensitive of modern plates, when handled with reasonable precautions, 
will astonish one. As a matter of fact most darkrooms are too dark 
and in the end not so safe as supposed, since it is necessary to work 
quite close to the light—not one of which is really safe. An overhead 
light will give an even, diffused light all over the room and there is 
no difficulty in finding articles which may be required. 

Lamps for indirect lighting are supplied by the Eastman Kodak 
Company (Fig. 23) and by Burke and James, but there is nothing to 


Fic. 23. Eastman Indirect Darkroom Lamp 


prevent the worker from making his own if he so desires, as the con- 
struction is quite simple. The light-box itself (see Fig. 24) is made 
of thin sheet iron and is 8x 10x 6% inches in size. There are two 
ventilators, one in one side and one in the bottom of the box, which 


must be made so as to prevent any white light from passing. The in- 


terior of the box is painted with a matte white or aluminum paint and 
the electric socket is placed so as to bring the filaments of the bulb 
nearly in the center of the box. The top is hinged so as to permit of 


~ THE CAMERA AND DARKROOM 51 


changing the safelight to suit emulsions of different character. The 
box is suspended from the ceiling by four chains or wires attached to 
each corner and the electric light cable is brought down from the wall 
tap which should be close by. To secure the full advantage of the 


To Wall Switeh 


bs 6 | 


Y | ______—.| -.--}} 


OE ‘ S Fi ° fs 
Wiring Diagram g 
Fic. 24. Design for Indirect Darkroom Lamp. 
(Krug, American Annual of Photography, 1922) 


light the ceiling should be a white matte; however, if this is not the 
case a sheet of Beaver board about four feet square and painted white 
may be fixed to the wall directly above the light. Under these condi- 
tions the illumination will cover an area about sixteen feet square so 
that additional lights will be needed for a large room. <A 25- or 40- 
watt bulb will supply sufficient light and very large bulbs should riot be 
used or the safelight will be melted. For gaslight papers one sheet of 
bright yellow or canary paper is sufficient. For bromide papers one 
sheet of orange glass will be safe or better still a No. o (bright- 
orange) Wratten safelight may be used. A sheet of ruby glass and 
one sheet of orange glass may be sufficiently safe for most non-color- 
sensitive emulsions but it is preferable to use such a screen as the 
Wratten Series 2, which will provide the maximum illumination that 
is safe for ultra-rapid or orthochromatic plates. For panchromatic 
plates the dark-green safelight (Wratten Series 3) should of course 
be used. 
A very convenient lamp for the development of prints or enlarge- 
ments or for either time or factorial development, where there is no 
occasion for examining the negative by transmitted light, is the 
pendant light (Fig. 25). As the safelights are interchangeable the 


52 PHOTOGRAPHY 


same light may be used for developing either plates or prints, the 
safelights being changed as the occasion demands. 

Among the many excellent types of lamps on the market for 
direct illumination the Wratten (Fig. 26) may be mentioned for the 


Fic. 25. Eastman Developing Fic. 26. Wratten Darkroom Safelight 
Lamp Lamp 


facility with which the safelights may be changed, for perfect ventila- 
tion, and for the uniform and safe illumination by using only reflected 
light. 


All the lamps which we have mentioned are for electric light only 


and certainly no one who has access to electric current will use any- 
thing else. Where gas or oil must be used the best plan is to place 
the light outside of the darkroom itself and arrange a holder for the 
safelight in the wall of the room. Where this cannot be done it is 
necessary to purchase one of the gas or oil lamps obtainable from 
dealers. In purchasing examine first the size of the lamp (5 by 7 
inches should be regarded as the minimum size for the safelight) ; 
second the arrangement for changing safelights for different plates ; 


and third see that the ventilation is well taken care of. Most com- 
mercial oil lamps are deficient on all of these points but more par-__ 


ticularly the second and third. Do not make the mistake of buying a 


cheap lamp, but examine carefully the various models and do not 


hesitate to pay a fair price for a well-built and efficient lamp which 
will fulfill your requirements. 


a ee rit oe 


THE CAMERA AND DARKROOM 53 


The Safelight.—The term safelight is in a sense a misnomer as 
there is really no such thing as a“ safe” light, for any light that is 
bright enough for the eye to observe will affect a plate if it is ex- 
posed to it sufficiently long. It is entirely a matter of time. It is well 
to remember that, regardless of the screen used for development, the 
plate should be exposed to the light as little as possible, only at the 
beginning of development to see if the plate is evenly covered and 
then towards the end of development to determine whén the plate 
should be removed, and the tray should be kept covered at all other 
times. Under these conditions it is possible to use a fairly bright 
light for development with comparative freedom from fog. 

In the earlier days of photography when plates were not nearly 
so rapid or as color sensitive as they are at the present time, the com- 
mon materials used were ruby glass and canary fabric. While both 
of these materials pass a considerable amount of active light, they 
were Satisfactory with the plates then in use and indeed are still 
widely used, but with the advent of the modern highly colored sen- 
sitive plate the ruby glass screen is giving way to the modern safe- 
light, composed of certain dyes, which are selected so that the com- 
bined transmission is that to which the plate for which it is designed 
is least sensitive. These screens are made by several manufacturers, 
in different series, for plates of different color sensitiveness, or they 
may be made at home, but as there are many excellent screens at very 
reasonable prices on the market it is a mistake and false economy to 
make one’s own. 

The Efficiency of Darkroom Safelights—With any darkroom 
screen we naturally wish to secure the maximum safety with the 
greatest visual intensity, or in other words the brightest light that is 
safe for the plate. Thus, if a plate is sensitive only to the ultra-violet, 
violet, and blue, and is insensitive to green and red, either a green or 
a red safelight might be used with safety. The most efficient of the 
two screens, and therefore the best choice for practical work, would 
depend upon the relation between the sensitiveness of the plate and 
the sensitiveness of the eye for any particular color. The visual in- . 
tensity is higher for the green than the red, but the latter has less ac- 
tion on the plate, so that with ordinary plates where a fair volume of 
light may be used, the red screen is used in preference to the green, 
but in the special case of the panchromatic plate, which is sensitive 
to practically the entire visible spectrum, so that only a very small 


54 PHOTOGRAPHY 


volume of light may be used, the green is chosen because of its greater 
visual intensity. 

The standard of safety adopted for the products of at least one 
manufacturer of screens is that no effect should be produced on the 
plate when it is exposed to the safelight at a distance of one meter 
for thirty seconds. In most cases, particularly the screens designed 
for developing (gaslight) and bromide papers, the sensitive material 
may be exposed to the light without danger of fog considerably longer, 
but most of the other screens will produce fog if rapid or highly 
color-sensitive plates are exposed to their action for much over a half 
minute. A good working test of safety is to place a plate facing the 
safelight and about two feet away and expose part of it for thirty 
seconds, leaving part of the plate unexposed by covering it with sev- 
eral coins, or a piece of black paper. If the plate shows signs of fog, 
after development in total darkness for the usual time, the light is un- 
safe for the plate and either a weaker light source should be used, the 
plate handled at a greater distance from the lamp, or the safelight 
should be changed to one which more nearly transmits light to which 
the plate is insensitive. 

Before leaving the subject of safelights we would again caution the 
worker about exposing plates to the safelight more than is absolutely 
necessary. This is particularly important when loading holders and 
before the plate is placed in the developing solution, as the sensitive- 
ness decreases as development proceeds, so that a light which may be 
safe for examining the plate when partly developed may be relatively 
unsafe for the same plate in the dry state. It is not a difficult matter 
to learn to load plates in the dark and it is far more satisfactory to do 
so. With time development there is no necessity for observing the 
plate at all unless greater or less contrast than the normal is desired, 
and then it is not necessary to examine the plate until development is 
nearly complete. The same applies to development by inspection. 
Particular care must be taken when the factorial method of develop- 
ment is used, since in this case the plate must be held rather close to 


the light at the beginning of development in order to observe the time 


of appearance of the image. 

- Trays, Tanks and Graduates.—Trays having the same dimensions 
as the plate used are really convenient only for special operations 
such as reducing, intensifying, etc., and it is far better to purchase de- 
_ veloping trays which will accommodate at least four plates at a time. 


t sg) 


THE CAMERA AND DARKROOM 55 


There is very little to choose between hard-rubber, porcelain, or steel 
enamel trays. All withstand all ordinary photographic solutions. 
While the first are expensive and easily broken, the second are rather 
expensive and also somewhat clumsy on account of their weight, and 
the last has the disadvantage that the enamel becomes chipped in time 
and exposes the metal to the action of the solutions. Composition 
trays are as satisfactory as any and if broken can be replaced with 
less outlay than any of the others. 

Now that tank development is so largely employed by both the 
amateur and the professional it may be well to say a word regarding 
_ the tanks for the development of plates and also those adapted to the 
use of roll film. Practically all of the tanks on the American market 
which are designed for the use of plates are made of nickeled steel 
and fitted with a waterproof cover so that the tank may be turned 
over on its end in order to agitate the developer and prevent uneven 
density. In several of the tanks on the market provision is made for 
pouring in the developer through a light-proof trap after the plates 
have been loaded into the tank. Then when development is complete 
the developer may be poured out through the same tap and replaced 
by pure water for rinsing and finally by the fixing bath. Thus all 
operations except the loading of the tank with plates may be done 
in full daylight, while it is quite an easy matter to load the tank with 
plates without a darkroom by using a changing bag. Other tanks 
have very ingenious loading devices which enable the tank to be 
loaded quickly without the danger of touching the sensitive surface or 
exposing the plate to the light very long. The Kodak film tank and 
the Rexo developing bag are examples of the all-by-daylight method 
of developing roll film and their use is too well known to require more 
than a brief mention. | 

The tanks for fixing may be of glass, hard rubber or composition. 
Perhaps the only warning needed in purchasing is to see that the top 
of the plate comes well below the top of the tank. It is not an uncom- 
mon matter to find tanks in which the plate reaches almost to the 
top of the tank and it is necessary to have the tank full of solution 
in order to cover the plate. With some tanks the grooves are so close 
together that it is almost impossible to grasp plates by the edges in 
removing. In such cases avoid filling the tank to its full capacity, or 
better still obtain a new tank having broader grooves. 

Only three sizes of graduates are required for all ordinary purposes, 


56 PHOTOGRAPHY 


although others may be useful at times. The sizes most generally re- 
quired are a one-ounce minim graduate, one of about 8 ounce capacity 
and a larger one of 16 or 32 ounce capacity. For the larger sizes the 
cheap tumbler graduates having pressed instead of engraved lines are 
sufficiently accurate. By paying somewhat more one can secure 
graduates having opaque graduations which can readily be seen in 
the darkroom, so that there is no uncertainty in preparing solutions 
while developing. Two special forms of graduates also require at- 
tention, viz.: the graduated beakers used by chemists which are prac- 
tically unbreakable, and the combined graduate and mortar and pestle 
which is extremely useful for powdering chemicals or crushing tablet 
developers, as the Tabloid or Scaloid products. 

It is essential that all trays, tanks, and graduates be kept thoroughly 
clean. Many of the stains and other defects which perplex the 


‘ amateur and trouble the professional at times are due to unclean trays. 


or other similar equipment. One of the most effective means of 
rapidly removing any ordinary chemical impurity from a vessel is 
by the use of a solution of potassium bichromate and sulphuric acid. 
The proportions are not so very important and the following will be 
found about right: 


Water to make. ..5 ..sas06s400 bee ol see 32 ounces (1,000 c.c.) 
Potassium bichromate 0 i. ¢; is92< «= 090 0. 4 ounces (125 gms.) 
Sulphuric acid (commercial). i240 so ee pee 4 ounces (125 c.c.) 


This is poured into the vessel and will act in a minute or so, when 
after a rinse or two and some swabbing with a tuft of absorbent cot- 
ton the tray or graduate will be chemically clean and available for any 
photographic operation. The solution should be kept in a glass-stop- 
pered bottle, as it will destroy a cork. 

Some Miscellaneous Workroom Features.—It is always advisable 
to keep the major part of the stock of sensitive materials on hand 
outside the workroom but provision ought to be made to keep those 
which are constantly in use where needed in the workrooms and in 
such quantities as may be required. To this end it is well to con- 
struct tight wooden cabinets to contain all plates, films and papers 
in general use not only in order that such materials may be kept in 
an orderly condition but to prevent injury by the moisture always 
present in the workroom. Such cabinets may also be provided for 
containing loaded and unloaded plate holders and for the weighing 
and mixing of chemicals. 


THE CAMERA AND DARKROOM 57 


The drying of plates and films is a matter of some moment, espe- 
cially in large commercial establishments where it is necessary to dry 
batches of plates or films at a uniform rate day in and day out re- 
gardless of the weather conditions outside. We illustrate in Fig. 27 
a small cabinet designed for plates or cut film and intended more par- 
ticularly for the serious amateur or small professional. For a larger 
establishment and where provision must be made for drying roll film 
more elaborate equipment is necessary. The size of the cabinet will be 
governed largely by the number of films handled in a batch and the 
number per day. It should be sufficiently large, however, to contain 


Fic.'27. Drying Cabinet for Plates and Films 


all the films it is likely to be called upon to handle at one time with- 
out undue crowding and ample space should be allowed for the cir- 
culation of air on all four sides and through the center. Either slid- 
ing or swing doors may be provided, while if desired one may place 
doors on two opposite sides, the films direct from the washing tank 
being inserted from one side and removed from the other after dry- 
ing. To secure a thorough and even circulation of air it 1s advisable 
to provide both the bottom and top of the cabinet with an air cabinet. 
Both the upper and lower walls of the bottom air cabinet should be 
made perforated, using % inch holes spaced two inches apart each 
way. The holes of the two walls should not coincide, however, or 
the purpose of the air chamber will be defeated. The bottom of the 
upper air chamber should also be perforated and provision made at 
the top for the installation of a suction fan to create a current of air 
through the cabinet. Provision for suspending the films will natu- 
rally depend upon the type of hangers employed and can be worked 


58 PHOTOGRAPHY 


out by the individual to meet his own particular case. Heat may be 
used for drying if a thorough even circulation of air is assured but 
not otherwise. The best results are obtained at 95 degrees Fahr. and 
a thermometer should be kept within the cabinet and in plain view 
to see that this temperature is not exceeded. 

Drying cabinets for roll film aré an article of commerce and may 
be obtained from a number of dealers should the worker prefer not 
to build his own. 

Such cabinets may also be used for drying prints on squeegee 
plates. Commercial apparatus for this purpose is also available. 


CHAPTER III 


PHOTOGRAPHIC OPTICS 


Introduction.—According to the theory now generally accepted, 
light is a wave motion in an elastic medium known as ether. The 
vibratory motion of the molecules of a body are communicated to the 
ether and a transverse wave spreads out in all directions with a velocity 
of approximately 300,000,000 meters per second. When this wave 
motion strikes the eye it produces the sensation which we term light. 

We distinguish between Juminous and illuminated bodies: the 
former radiate light of themselves, the latter are simply bodies on 
which the rays from a luminous source fall. Among the former are 
included the sun, all artificial lights as oil, gas, electric and magnesium, 
and every body heated above a certain point. Illuminated objects com- 
prise all bodies in the unobstructed path of rays from a luminous 
source. ‘The intensity of illumination on an illuminated body depends 
upon the strength of light at the source, the distance of the illuminated 
body from the light source and the density of the intervening medium. 

While light waves spread out in all directions from the source, the 
vibrations which reach any point travel a straight line joining that. 
point and the source. This is known as the rectilinear propagation of 
light. 

A cone of rays from a luminous point is known as a light-pencil, 
and is said to be homocentric to all other pencils which proceed from 
the same luminous point. The central line of this light-pencil is 
termed the axis. Ii we follow the rays from the source they are said 
to be divergent; in the opposite direction, convergent. 

Refraction of Light.—In a homogeneous medium the path of a ray 
of light is always straight, but the passage of the ray from one medium 
to another, in general, alters both its velocity and its direction. The 
alteration in the direction of a ray upon passing from one medium to 
another is known as refraction. The angle which the incident ray 
makes with the normal to the surface at the point of separation of the 
two mediums is known as the angle of incidence, while the angle of 
the refracted ray to the normal is known as the angle of refraction. 

A ray (see Fig. 28) a moving in a medium 1 meets at D the surface 

59 


60 PHOTOGRAPHY 


cd separating the medium 1 from a denser medium 2. Upon striking 
the surface cd, the ray is divided into two rays be and bf, the former 
the reflected and the latter the refracted ray. It will be observed that 
both the reflected and the refracted ray lie in the same plane as the 
incident ray. The angle of the reflected ray is equal to the angle of 
the incident ray, so that the angle of incidence equals the angle of re- 


Fic. 28. The Principles of Refraction 


flection. When the ray passes from one medium to another of greater 
density, as from air to glass, the refracted ray bf is bent towards the 
normal, but when passing from a dense medium to one of lesser den- 
sity the path of the refracted ray lies away from the normal. 

The angle of deviation of a ray upon passing from one medium to 
another is known as the index of refraction. Since refraction is due 
’ to a change in the velocity of light the refractive index of a medium is 
equal to | 


the velocity of light in air or a vacuum 
the velocity of light in the given medium 


The amount which the refracted ray deviates from the direction of 
the incident ray, or the index of refraction, can be determined from 
the size of the angles ab] and bf’. Thus the angle bf ==abl— bf’. 
Or, where 7 is the angle of incidence, r the angle of refraction and D 
the deviation, the value of D is obtained by the following equation: 


D= (4-1). 


Thus far we have only considered refraction at one surface. When 
we have a plate of a dense medium, as glass, a ray of light is refracted 
both upon entrance and also upon emergence. Let ABCD (Fig. 29) 
be a block of glass with sides 4B and CD parallel. The ray of light 


PHOTOGRAPHIC OPTICS 61 


ab strikes the surface AB at b. This ray is refracted from the normal 
on entering the denser medium and proceeds to c. Upon passing from 
the surface CD into a medium of lesser density, refraction again occurs 


Fic. 29. Refraction in a Medium with Parallel Sides 


but this time towards the normal. Since AB and CD are parallel, the 
normals are parallel and the emergent ray is parallel to the incident 
tay. There is a lateral displacement but not a change in direction. 
However, if the sides are not parallel, as in a prism, the course of 
the ray is altered as illustrated in Fig. 30. If i is the angle-of inci- 


Fic. 30. Refraction in a Prism 


dence ABN, r the angle of refraction NBC, then the deviation is 
@—1r). (2) 


In like manner if 7’ and 7’ are respectively the angles of incidence NCD 
and refraction BC, then the refraction at C is equal to (7’ —r’) and 
the total refraction (D) is represented by 


D=t-7)-(r—-7). 

Dispersion.—So far we have considered only monochromatic light 
or light having one definite color, but in practice we do not deal with 
monochromatic light but with daylight, or at least a light source hav- 
ing a wider range of emission than one narrow band of the spectrum. 

6 


62 PHOTOGRAPHY 


When white light is passed through a prism the different rays are not 
all refracted to the same degree but according to their refrangibility ; 
those of short wave-length, as blue and violet, being refracted to a 
greater degree than those of longer wave-length, as orange and red 
(Fig. 31). The angular separation between the constituents of a ray 


on OD 


Fic. 31. Dispersion in a Prism 


of composite light produced by refraction of the ray in passing 
through another medium is known as dispersion, 

Dispersion was formerly thought to be dependent upon refractive 
power so that substances having a high refractive power necessarily 
had a high dispersive power. This is now known to be incorrect and 
it has become possible to prepare glasses with high refractive power 
and low dispersion and vice versa. Dispersion results in what is 
termed chromatic aberration, the correction of which is of considerable 
importance in photographic objectives and will be fully treated in the 
following chapter on the aberrations of the objective. 

Lenses and Image Formation,—There is a very close similarity be- 
tween a lens and a combination of prisms and in fact we may consider 


mi 


Fic. 32. Principal Forms of Simple Lenses 


a lens as a prism having an indefinite, or infinite, number of sides. 
Single lenses are divided into two classes according to whether they 
are diverging or converging; the former are known as negative lenses, 
the latter as positive. The principal forms of converging and diverg- 
ing lenses are shown in Fig. 32. It will be observed that the former 


Pag OGRAPHIC- OPTICS 63 


are thicker at the center than at the edges, while the latter are thicker 
at the edges than at the center. 

The position and character of the image formed by a positive lens 
will be seen from Fig. 33. In this case AB represents an object 


) 5 


B + 


Fic. 33. Image Formation with Positive or Converging Lenses 


placed at a distance from the lens which is much greater than the focal 
length of the lens. For purposes of illustration the course of only 
three rays proceeding from the object points will be shown. The 
three rays from A pass through the lens and are refracted so that they 
intersect one another at A’, while the rays from B intersect at B’ ina 
similar manner. Likewise the rays from any point on the line AB on 
passing through the lens will be refracted and form a corresponding 
point on the line A4’B’.* The image in this case is real and inverted. 
The point at which the rays begin is termed the object point and the 
point at which they intersect after having passed through the lens is 
known as the image point or the focal point, while the distance from 
a point in the lens known as the nodal point to the focal point is known 
as the focal length. 

From Fig. 34 it will be observed that a negative, or dispersing, lens 


J 


‘a 


Fig. 34. Course of Light Pencils through a Negative Lens 


forms no real image but only a virtual one since the focal point lies 
between the lens and the object. 

Image Formation according to the Gauss Theory.—We owe to 
Gauss the conception -of the nodes and nodal planes of a lens, also 
called principal or Gauss points after their discoverer. If the positions 
of these are known, the image can be constructed from the object 


1 We are assuming, of course, a fully corrected lens. 


64 PHOTOGRAPH 


without knowing anything about the actual course traversed by the 
rays in their passage through the various glasses. 
In Fig. 35 are shown the nodes N,N, in a single lens. A ray of 


Fic. 35. Image Formation According to the Gauss Theory 


light entering at an angle so that it reaches the node at NV, (node of 
admission) acts as if it was carried parallel to the axis to V, (node of 
emergence) after which it continues on in a straight line. If we 
imagine the planes P,P, andP,P, passing through the nodes N, and 
N, parallel to the axis, we will see that a ray of light, as ab, parallel to 
the axis on reaching the lens passes undeviated to the plane of emer- 
gence and is there bent so that it passes through the focal point. If 
the lens is reversed the node of admission becomes that of emergence 
and vice versa. Thus every lens may be said to have a front focus and 
a back focus, the former measured from the node of admission, the 
latter from the node of emergence, to the focal point. 

In Fig. 36 N, and N, are the nodes of a lens, P,P, and P,P, the 


=== Eyefef/R- ~~ += =f === 
i 


Fic. 36. Image Formation According to the Gauss Theory 


corresponding nodal planes, F, and F, are the front and rear foci re- 
spectively, so that F,N, is equal to F,N,. Let ab represent an object 
having its lowest point on the lens axis bb. The position of point a 
in the image may be found by drawing two rays AP, parallel to the 
axis and thence through the rear focus F,, the other ray through the 


PHOTOGRAPHIC OPTICS , 65 


front focus Ff, to the plane of admission P, and thence parallel to the 
axis. The meeting point of these two rays (a,) is the image point of 
a. Any other point may be found in like manner, and hence if the 
position of the nodal planes is known the rays can be traced through 
the lens whether the curves or glasses of the objective are known or 
not. 

The Position of the Nodes.—The positions of the nodes and nodal 
planes in a single lens vary with the type of lens. In Fig. 37 the posi- 


(| Na:; Na: Na: Nai Na Na| | Na\ Wai Na‘! 
Na eal A | 
: | 
| | 
ATTA AIM LAY 
| Na \|Na | Wal Wa ila |Wal Wa | Nal! Na \'Na 


Fic. 37. Position of the Nodes in Common Forms of Simple Lenses 


tions of the nodal planes are shown for a number of the common types 
of single lenses. ‘The light is assumed to be passing from left to right 
in the direction indicated by the arrow which also indicates the axis 
of the lenses. Nand N, represent the nodes of admission and emer- 
gence respectively. When the curves are equal the nodes are centrally 
placed ; when the curves are unequal the nodes lie nearer the side hav- 
ing the greatest curvature. When a lens has a plane surface one node 
lies at the vertex of the convex surface, while in the case of a meniscus 
lens one node lies outside of the lens, or “ free.’ In combinations of 
lenses the positions of the nodes vary considerably. They may lie 
within the lens itself near the diaphragm, in front of, or behind the 
lens. When the nodes are situated a considerable distance in front of 
the lens we secure a great focal length with a short distance between 
the rear of the objective and the ground-glass. This is the principle 
upon which the teleobjective is based.? 

The Principal Focus of a Lens—Focal Length.— When the object 
is at an infinite distance from the lens so that the incident pencils of 
light are parallel on entering the lens the focus of the emergent 
pencil constitutes the principal focus of the lens. Since either side 
of the lens may face the subject, there are two principal foci, both 
of which are equidistant from the node of emergence but on op- 
posite sides of the lens. 


2For a more complete discussion of the nodes and their action see Piper, 
The First Book of the Lens, pp. 31-32 and 45-49. 


66 PHOTOGRAPHY 


In Fig. 38 A and B represent the outer rays of a parallel pencil of 
light incident on the nodal planes Na of a lens L. On leaving the 
node of emergence N- the rays are converged to a point FP, This 
point F represents the principal focus of the lens and the distance 
NF constitutes the focal length. If the lens is reversed with respect 


aa 
Gove 
= a, 


_——. 


ig gees 


yf 
eee me owen - w= focalheye—— ca waa t---------- foew herWh ara e SP oes ~ Soe 
Pie 


Fic. 38. Focal Length 


to the subject, the node of admission becomes that of emergence and 
vice versa, and F is formed on the other side of the lens at precisely 
the same distance from the node of emergence as before. 

In considering parallel pencils of the same diameter, the deviation 
of the outermost rays of the pencil is the same when comparing lenses 
of the same focal length, but if lenses of different focal lengths are 
compared the deviation is greater with the lens having the shorter 
focal length. The deviating power of a short focus lens is therefore 
greater than one of long focus and consequently it is said to have 
greater focal power. ‘That is, it has the power of converging the © 
rays of a pencil of light to a greater degree than one of longer focus, 
or less focal power. From the viewpoint of the user of photographic 
objectives, however, the term focal power is without practical signifi- 
cance. 

Back focus is the term applied to the distance between the image 
and the node of emergence of the nearest single lens of the combina- 
tion. It is a relative term and as a rule indicates roughly the ex- 
tension of the camera required. The back focus is always less than 
the focal length of the lens excepting with lenses which have their 
nodes behind the back combination. It is not of much importance 
except with telephoto lenses. 

Focal Length and Size of Image.—Assuming a concrete distance 
between lens and subject, the greater the focal length of the ob- 
jective the larger the size of the image. Except with very near ob- 


PHOTOGRAPHIC OPTICS 67 


jects, the size of the image varies directly as the focal length of the 
objective. Thus with a given distance between lens and subject the 
image produced by a twelve-inch lens is twice as large as that pro- 
duced by a lens with a focal length of only six inches. Stated dif- 
ferently, to obtain a given size of image the distance between the lens 
and the object decreases as the focal length. 

While the perspective produced by a lens is always scientifically ac- 
curate, regardless of the distance between the lens and the subject. 
it does not necessarily accord with our established idea of perspective. 
Owing to the fact that the eye is a long focus instrument and accus- 
tomed to viewing objects at a relatively great distance, photographs 
made with the lens very close to the subject appear to possess violent 
and unnatural perspective to the eye, so that the result is unsatis- 
factory to our zsthetic sense even though it may be scientifically ac- 
curate. 

Perspective is determined entirely by the distance between the lens 
and the subject and is independent of the focal length of the lens, 
but since in practice when using a lens having a short focal length the 
tendency is to get close to the object in order to secure an image of 
satisfactory size, short focus lenses have gained the reputation of 
producing violent and disagreeable perspective. There is, however, 
no actual foundation for this, for if the distance between the lens 
and the subject is the same the perspective of identical sections of 
the subject will be the same. Thus if two plates of the same size 
are exposed, using two lenses of different focal lengths at the same 
distance from the subject, one plate will include very much more of 
the subject than the other. However, if this latter is trimmed so as to 
include the same portion of the subject as the other the two prints 
will be identical, except in size. Consequently a lens of long focal 
length gives superior perspective only because the distance between 
the lens and the subject must be greater for an image of a given size. 

The choice of a suitable focal length is governed by the size of the 
plate and the requirements of the subject. In certain cases short focus 
objectives must be used in order to include all of the subject from 
a given viewpoint. In such cases violent and unnatural perspective 
cannot be avoided and must be accepted. Generally, however, there 
are no such limitations and the focal length is, within certain limits, a 
matter of convenience. For all general work it is well to choose a 
lens the focal length of which is equal to, or slightly greater than, 


68 PHOTOGRAPHY 


the diagonal of the plate. For pictorial work or portraiture, how- 
ever, longer focus lenses are desirable and for such work lenses hav- 
ing a focal length two, or even three, times the diagonal of the plate 
are used.® 

Angle of View.—The angle of view of any objective is the ire 
subtended by two lines drawn from the node of emergence to the. 


64 
ooo ooo 
Att tA TT TY SEs 60 


POA 
Pere “and 
VIF IZ 1 

AAP 

ff & 


f) 


7 [A 


AND 
CCAS SS 


*Diaaonal of Plate in-Inches 


a 
7 
Fi 
7 
i 
et 


PREV 


SSa a ar: 


a 


atl 


| ot] 


aN 
AGN 


TA TY 


OCCA NEC NCES 


c 
| 
| 

20} | Il 


SQlrNVIMITI LL Sl|o\q 


_ Focal Length tn Inches 
Fic. 39. Table for the Calculation of Angle of View 


AEA SASS 2A Se 
SEBS Nea 


corners of the plate in use. The shorter the focal length of the lens 
in relation to the size of plate the greater the angle of view and the 
more of the subject included from a given viewpoint. | 

3 For a complete discussion of the fundamentals of perspective see paper by 


J. C. Dollman in the Photographic Journal, 1923, 63, 315. The same paper 
will also be found in the British Journal of Photography, 10923, 70, 411. 


el 


PoWlOGRAPHIC OPTICS 69 


It is often of advantage to be able to calculate the maximum angle 
of view for a given lens on a certain sized plate and to render this 
a simple matter we have reproduced a chart by which this information 
can be simply and rapidly secured. 

The maximum angle of view possible with any lens Heneas upon 
the relation between its focal length and the size of the plate which 
it will cover with satisfactory definition. For a given size of plate, 
the shorter the focal length of the lens the greater is the angle of 
view, while on the other hand, the larger the plate which a lens of 
given focal length will cover with sufficiently critical definition the 
ereater is its angle of view. Thus a lens of six inches focal length 
will include a much greater angle if used on a 5x7 plate than the 
4x5 for which originally designed. Most lenses, however, particu- 
larly if of high speed, will not cover satisfactorily a plate much larger 
than that for which they are designed unless considerably stopped 
down. Some lenses, such as those built on the “ Tessar ” construc- 
tion, do not increase in covering power to any extent when stopped 
down and naturally their angle of view is limited by the plate for 
which they are designed to cover. A lens made to cover with sat- 
isfactory definition a field large in proportion to its focal length is 
termed a wide angle lens. While no definite rules exist for classifying 
lenses according to angle, lenses including an angle of less than 45 
degrees may be considered as narrow angle; those including angles 
from 45 to 75 degrees as medium angle, and any lens including an 
angle above 75 degrees as a wide-angle objective. _ 

Conjugate Focal Distances——When the distance of the subject is 
less than infinity, any object point and its corresponding image point 
are termed conjugate foct, and the distance from the object point to 
the nodal plane of admission and that from the image point to the 
nodal plane of emergence form what are termed the conjugate focal 
distances. ‘These distances are interdependent and connected by a 
certain formula. 

Let u represent the distance of the object, vw that of the image, and 
f the focal length of the lens. Then 


70 PHOTOGRAPHY 


If the values of any two of the above terms are known it becomes 
a simple matter to find the other from the following: 


vu 
Bs v+ Hs 
2p : 
uU is amar’ . 
Cette 
e bof 
From the formula 
esi 
fo ia Se 


it will be evident that when the distance of the object and that of 
the image are equal (i.e. w equals v) both distances are equal to 2f. 
The image is thus the same size as the object and the term symmetric 
foci, applied by Prof. Thompson, is apt. 

The linear ratio of the object and the image is expressed as 


Distance of image (v) 
Distance of object (x) 


but for purposes of calculation it is much simpler to express the 
ratio in terms of uw and f, or v and f (f representing the focal length — 
of the lens). From Fig. 36, representing the principles of image 
formation according to the Gauss theory, it will be seen that triangles 
abF, and F,N,P, are similar, whence 


N2P2 x FiLNe ; 
ab bFs 
By construction, N,P, is equal to a,b, and F,N, is equal to f and 
bF, equals v —f. 
Therefore, 


Image _ Rez ‘jj 


Object rer 
From triangles a,b,F, and P,N,F, 


Image_ p_ af, 
Oiect 


PRHOLOGRAPHIC OPTICS 71 


Extra Focal Distances.—From the last two formule it is evident 
that calculation is made much simpler when the distance of the ob- 
ject or the image is reckoned from the front or rear focal point re- 
spectively. These distances which are therefore u — f and v—f are 
known as the extra focal distances. The extra focal distance of the 
object is denoted by E, and that of the image by Ex. 

Therefore, 


Ey or LR, 


ip a = 5 RN 
i,=u—-f= R 
In other words, the extra focal image distance is equal to the focal 
length of the lens multiplied by the ratio of image to object, while the 
extra focal object distance is equal to the focal length divided by the 
ratio of image to object. 
Calculations of the conjugate distances can be made quite simply 
by considering the distance between the image and the object to be 
divided into five separate distances. These are: 


1. The extra focal distance—focal length multiplied by the number 
of times of reduction or enlargement. 

. One focal length. 

. Lhe nodal space—generally negligible. 

. One focal length. 

. The extra focal distance—focal length divided by the number of 
times of reduction or enlargement. 


im & WwW bd 


In the case of reduction (1) and (2) are on the object side of the 
lens, but when enlarging (5) and (4) are on the object side of the 
lens and (1) and (2) on the image side. When copying same size 
(1) and (5) are alike and are equal to one focal length, so that the 
whole distance from object to image is equal to four focal lengths, if 
the nodal space be neglected. 

The above are the fundamental formule covering the relation of 
the conjugate foci when enlarging or reducing and almost any re- 
quired calculation relating to the sizes and distances of object and 
image may be worked out from the formule above.* 

Theory of Depth of Focus.—In explaining the theory of depth of 

4 Numerous formulz for the calculation of the various problems pertaining 


to scale in optical reproduction were given by Mr. George E. Brown in the 
British Journal of Photography for 1921, 68, pp. 702-705. 


72 PHOTOGRAPHY 


focus, more properly termed depth of field, use will be made of the 
illustration developed by Moritz von Rohr.® 

In Fig. go let O,0,0, represent an object, parts of which are at 
different distances from the lens L. For the sake of simplicity the 
latter is represented without the nodes necessary for a complete rep- 


Fig. 40. Graphic Illustration of the Principle of Depth of Focus. (Von Rohr) 


resentation of its action. Suppose the lens to be focused on O,, a 
point image J, will then be formed on the focussing screen or sensi- 


tive plate. With the focussing screen, or the sensitive plate, at this 


point it is clear that the image of O, is formed at J, behind the screen 
and that of O, at J, in front of the screen. In other words the posi- 
tion of the image point varies with the distance of the object point 
and the lens cannot produce a point for point image upon a plane 
subject unless the subject itself is a plane surface. When this is not 
the case, rays proceeding from object points nearer to, or farther 
from, the lens than the plane focussed for do not produce a point 
image on the focussing screen but instead a circular disk. However, 
owing to the fact that the eye has its limits of definition and cannot 
appreciate critical sharpness, these disks may appear as point images 
to the eye if their diameter is less than a certain size which is termed 
the circle of confusion. The circle of confusion thus represents a 
circle of the largest diameter possible without producing perceptible 
unsharpness to the eye. The largest circle which will appear as a 


5 Zur Geschichte und Theorie des Photographischen Teleobjectiv. 1897. Also 
Eder’s Jahrbuch for 1906. 


Oe ay a a d 


Pe OGha rei OPTICS 73 


point to the eye is a variable dimension depending upon the distance 
from which the photograph is viewed. It is found that at a distance 
of 12 inches, which is a normal viewing distance for prints up to ap- 
proximately 8 x 10, a circle with a diameter of not more than 1/100 
of an inch will appear as a point to the normal eye. Consequently this 
has to a certain extent been adopted as a standard for satisfactory 
definition in photographic work. It is to be noticed that this value 
applies to the finished print but not necessarily to the original negative. 
For instance when a negative is enlarged the standard requires to be 
reduced in proportion to the degree of enlargement in order that the 
circles of the enlarged print may not exceed the maximum permissible 
diameter. Where negatives are to be subsequently enlarged the circle 
of confusion should not exceed 1/250 of an inch in diameter and in 
cinematography still less departure from critical sharpness is allow- 
able: in this latter case the diameter of the circle of confusion allow- 
able is about 1/600 of an inch. Hence, the better plan of expressing 
the circle of confusion is in terms of the viewing distance, as adopted 
by Continental writers, rather than the adoption of an arbitrary stand- 
ard. | 

According to the latter view, the permissible circle of confusion is 
not a definite value, but a variable, which depends upon the distance 
from which the print is viewed. It is found that any object, regard- 
less of size, will appear as a point to the eye when viewed at a dis- 
tance equal to, or greater than, 3,400 times its diameter. This is 
equivalent to the very small angle of I minute (or a 5,400 part of a 
right angle). In order to be sharply perceived the angle must be 
at least 5 minutes, hence objects subtending angles of 1’—4’ are per- 
ceived more or less distinctly and appear practically sharp to the eye. 
This view of the variable standard of the admitted disk of confusion 
has much to commend it, and in view of the widespread practice of 
enlarging from small negatives it would probably be better if it were 
universally adopted and the older method of an arbitrary standard 
abandoned.® 

Factors Controlling Depth of Focus.—The factors which control 
depth of focus are: 


1. The focal length of the objective. 


6 For a full discussion of the two methods and the factors on which the same 
are based see “ Theory and Practice of Depth of Focus” by George E. Brown 
in the British Journal of Photography, 1922, p. 476 et seq. 


74 PHOTOGRAPHY 


2. The aperture of the lens. 

3. The distance of the plane in sharp focus. 

4. The permissible diameter of the circle of confusion. 
5. The presence of spherical aberration. 


The extent of field in sharp focus, other conditions remaining con- 
stant, is greater the shorter the focal length of the lens. The depth 
of field varies inversely as the square of the focal length, if near ob- 
jects be excepted. It is for this reason that fixed-focus cameras and 
cameras focussed by scales are generally fitted with lenses of the short- 
est focal length consistent with the size of image required for the plate 
in use. 

Increasing the rapidity of the lens reduces quite rapidly the depth of 
focus. Thus with a lens of six inches focal length focussed on infin- 
ity, all objects up to 20 feet of the camera will be sharp if the lens is 
used at F/16. If the lens is opened. up, however, say to F/4, then 
the depth of focus will only extend from infinity to within 75 feet of 
the camera. The difference in the extent of field in sharp focus at 
different apertures is even more noticeable when a plane comparatively 
near the camera is focussed for. 

From what has already been said it will be evident that depth of 
focus in any given case will depend considerably upon the standard 
of definition adopted as satisfactory. Thus the depth of focus will 
be much greater if a circle of confusion equal to 1/100 of an inch is 
accepted as satisfactory in place of 1/250 inch or less. For contact 
prints up to 8 x Io the former standard is satisfactory but when 
smaller negatives are enlarged up to that size the latter standard, or an 
even higher value, must be adopted if the resultant enlargement is to 
appear critically sharp on inspection at the normal viewing distance 
of 12 inches. 

The presence of spherical aberration increases the apparent depth of 
focus since it destroys critical definition and as a result there is no 
clear dividing line between that portion of the subject which is sharp 
and that which is unsharp. An excellent example of this is seen in the 
modern soft-focus objectives which have greater depth of focus than 
other objectives of the same focal length and aperture because the 
difference between the portions in focus and those slightly distant 
from the plane focussed on is not so readily noticeable, and also 
because a circle of confusion greater than 1/too of an inch is ac- 
cepted as permissible. If the depth of such an objective be considered 


Pov LOGRAPEIC: OPTICS 79 


from the latter viewpoint, it is quite possible that it might have no 
depth whatsoever since the circles of confusion would nowhere be less 
than 1/100 of an inch in any part of the field. 

Depth of focus has always been, and still is by the optical world at 
large, considered to be controlled entirely by the focal length and the 
aperture of the objective, other conditions being assumed constant, 
but quite recently Paul Rudolph has disputed this and claims for his 
latest lens, the Plasmat, a greater depth of focus than other lenses of 
the same optical constants. It is generally agreed, however, that 
Rudolph has not satisfactorily proved his case and that the greater 
depth of his Plasmat is due to other factors.’ 

The Intensity of the Image.—The intensity of the image formed by 
the lens is naturally a matter of considerable importance since on it 
depends the time of exposure required to impress the image on the 
sensitive film or plate. It is evident that where other conditions are 
constant the times of exposure with any two lenses will be in inverse 
proportion to the intensity, or brilliancy, of the image which they 
project on the sensitive material. Neglecting for the moment losses 
in the incident light due to absorption or reflection by the glasses of 
which the lens is composed, the intensity of the image on the axis is 
the volume of light admitted divided by the area over which it is 
spread. ‘The intensity of the image at points removed from the axis 
is naturally dependent to a certain extent upon the optical perform- 
ance of the lens, but the intensity of the image on the axis is governed 
solely by the volume of light passing through the lens divided by the 
area over which it is spread in order to form the image. 

The volume of light admitted is of course dependent upon the area 
of the diaphragm aperture. Since the areas of circles are to one an- 
other as their diameters squared the volume of light transmitted by 
any two lenses is directly proportional to their diameters squared. 
Thus the volumes of light passed by any two lenses having an aperture 
of d, and d, are to one another as d,? : d,’. 

In case the light from d, and d, is spread over the same area, in 
other words the two lenses form an image of the same size, the in- 


7 Graphical methods of determining problems relating to depth of focus have 
been described by Lee. See Phot. J., 1922, 62, 239; also by H. C. Browne in the 
British Journal of Photography, 1923, 70, 775. 

For a graphical method of determining the distance on which to focus in 
order to obtain the maximum depth of focus see paper by Capt. S. M. Collins, 
Brit. J. Phot., 1920, 67, 659, 676. 


76 PHOTOGRAPHY 


tensity of the image will be directly proportional to d,? and d,*. Thus 
the intensity of the image is proportional to d?. ‘ 
Suppose, however, a lens with an aperture of diameter d, (Fig. 
41) has a focal length of f,. If O be a luminous point in an object 
at such a distance from the lens that the incident rays are practically 


Fic. 41. Intensity of the Optical Image. (Brown) 


parallel, then the image will be formed at the principal focus of the 
lens f, and may be represented by F,F,. Now suppose this lens is 
replaced by another having the same aperture but with a focal 
length twice as great as f, so that f,==2f,. Then the image F,F, 
will be twice as large as that of F,F,. Hence 


Diameter Diameter 
Fi Fy FoF, . 5 
f h ( 
But the areas of the images F,F, and F,F, are to one another as the 
squares of their diameters, hence we may write (1) as follows: 


(Fifi)? | (FoF)? | da) 
Siu) ae 
Thus the areas of F,F, and FF, are proportional to the correspond- 
ing focal lengths squared. As the aperture, d, is constant, the inten- 
sity of the image is thus inversely proportional to the focal length 
squared, or f?. 
Hence we have 
volume of light proportional to d? ; 
area of image proportional to f? ’ 


the intensity of the image, then, is represented by the expression 


d?/f?. (3) 


If this is written (d/f)?, the d /f expresses the ratio between the aper- 
ture and the focal length and is termed the aperture ratio. 


PHOTOGRAPHIC OPTICS 77 


~ Speed of Lenses—Systems of Diaphragm Notation.—The aperture 
ratio d/f affords a convenient means of expressing the speed of lenses. 
It is evident that the intensities of two lenses are to one another as the 
squares of their aperture ratios. Thus if the aperture ratios of two 
lenses are represented as d,/f, and d./f, respectively, the relative in- 
tensities are as 


(di/fi)? + (da/fe)?. 


Hence if the intensities of the image are expressed by the aperture 
ratios d/f, the relative intensities may be found by squaring both 
ratios and finding the relation between the squares. 

It is not customary, however, to mark lenses according to the in- 
tensity of the image, but according to the exposure required, which is 
of course in inverse proportion to the intensity, for as the brilliancy 
of the image is increased the time of exposure is proportionately de- 
creased. Accordingly the diaphragm numbers on lenses represent not 
the intensities but rather the reverse of these or the “slowness” of 
the lens. 

In 1881 a committee of the Royal Photographic Society of Great 
Britain advised that an aperture ratio of 1:4 be adopted as standard 
and.a series of aperture ratios chosen so that each successive aperture 
would have an area one half that of the preceding, in order that the 
exposure required for each successive aperture be double that for the 
one immediately preceding and avoiding altogether the necessity of 
squaring the aperture ratios in order to find the relative exposures for 
the various apertures. 

Since a circle of one half the area has a diameter ates to 1/\/2 the 
series of ratios becomes 


ee ia——e— — 


foe 50 O° 1 T.3- 1G: 22.6 ‘32° 45°64 


or in terms of i/d 


De Roo 113 + 1622.6 632 45164 


Relative exposure required 
Poti <1) 321" 6472128" 286 


It was further advised that in the case of lenses having a maximum 
aperture ratio intermediate in value between any of the above the 
F/number obtained by dividing the aperture by the focal length be 


marked upon the mount. Accordingly we have lenses bearing F/num- 
“ : 


78 PHOTOGRAPHY 


bers of F/4.5, F/4.7, F/6.3, F/6.8, etc. The relative exposure re- 
quired for any of these F/numbers may be found by squaring it and 
the nearest //number in the series and dividing one by the other. 

_A number of other systems of diaphragm notation have been pro- 
posed, some of which have been adopted by single manufacturers for 
short periods of time, but these have now disappeared and the F/sys- 
tem as developed by the Royal Photographic Society has been adopted 
by practically every manufacturer of any inp in the world and 
its use is practically universal. . 

Tables showing the comparative values of several of the minor sys- 
tems of stop notation with the standard f system will be found in the 
current issue of the British Journal Almanac.. 

Effective Aperture.—In the above sections the area of the aperture 
has been assumed to be the same as the actual area of the diaphragm, 
d. This is correct where the diaphragm is in front of the lens as in 
such cases the diameter of the pencil of light which can pass through 
the lens is obviously equal to the diameter of the diaphragm. In the 
case of photographic lenses having the diaphragm between the com- 
ponents, the diameter of the pencil of light which passes through the 
lens may be much greater than the actual diameter of the aperture, 
owing to the fact that the front lens acts as a condenser and converges 
the incident rays so that a pencil larger than.the actual aperture may 
pass through. The diameter of the pencil which is converged so as 


to pass through the diaphragm constitutes what is termed the effective 


aperture. 

A practical illustration of the difference which may occur between 
the effective and the actual aperture due to the converging action of 
the front component was given some years ago by Dr. Zschokke for the 
Goerz Dagor, a double, symmetrical anastigmat the halves of which 
are idenical. When the front component is used alone in its normal 
position in front of the diaphragm the effective aperture due to the 
“coning” action of the front lens is F/11.3. When the rear com- 
bination is used behind the diaphragm the effective aperture coincides 
with that of the diaphragm and is reduced to F/13.6. The brightness 
of the image is therefore nearly one and a half times greater in the 
first case than in the second, a difference due solely to the converging 
action of the front component. 

The difference in the effective and the actual aperture may be even 
greater with lenses of different design. Thus in the Aldis F/6 lens 
the actual diameter is only about .87 of the effective aperture. 


PHOTOGRAPHIC OPTICS 79 


It is evident upon further consideration of the matter that the effec- 
tive aperture will vary according to which end of the lens faces the 
subject unless both components are identical. In the case of the Aldis 
F/6 the components are dissimilar and the aperture when the lens is 
reversed, with regard to the subject, is only F/6.9 or for all practical 
purposes F/7. . 

With a lens having the diaphragm in front, the effective aperture 
varies to a certain extent with the distance of the subject, owing to the 
fact that the incident pencil of light is divergent rather than parallel. 
This effect, known as inconstancy of aperture, is so small in most cases 
that for practical purposes it may be disregarded. 

It is evident that it is the effective, rather than the diaphragm, aper- 
‘ture which must be considered in calculations regarding the intensity 
of the image. Hence it is more accurate to say that fhe aperture 
ratio d/f is 

the effective aperture 
focal length of lens 


than simply the aperture divided by the focal length as before. 

Visual methods of determining the effective aperture have been de- 
scribed by C. Welborne Piper and by Jobling and Salt,® but perhaps 
the method most generally useful consists in focussing the lens on a 
very distant object and replacing the ground-glass by a card, or other 
thin opaque material, pierced with a pinhole on the optical axis. A 
lighted candle, or electric bulb, is then placed next to the pinhole. It 
is evident that since the latter is at the focus of parallel rays, light from 
a source in this position will emerge from the lens as a parallel pencil 
of a diameter equal to the effective aperture. The circle correspond- 
ing with the effective aperture may be obtained by pressing a scrap of 
bromide paper against the lens and exposing, or by outlining the same 
on a sheet of thin paper. 

Loss of Light in Lenses Due to Absorption and Reflection.—The 
statement is often made that all lenses require identical exposures when 
used at the same relative aperture, but this is not strictly true as owing 
to the differences among lenses in the amount of light lost from ab- 
sorption and reflection by the glasses of. which the objective is com- 
posed, the actual intensity of the images may vary considerably, not- 
withstanding the fact that all are marked with the same F/number. 


8 Jobling and Salt, Brit. J. Phot., 1922, p. 108. Piper, Brit. J. Phot., 1917, p. 
272. 


80 PHOTOGRAPHY 


There is thus theoretical foundation for the statement sometimes made 
by experienced workers that a certain lens is faster than another of 
different design, although both are marked at the same F/value. 
While the loss of light from these causes may not be serious in the 
majority of work, it is evident that it will be a matter of importance 
where very short exposures must be made under unfavorable condi- 
tions and hence it is the aim of the designer to keep the loss of light 
from these causes as low as is possible with good correction. 

The amount of light lost by reflection from the glasses of the ob- 
jective is greater than might be at first supposed. There is no ap- 
preciable loss from reflection at a cemented surface, but any increase 


in the number of glass to air surfaces increases the amount of light 


lost by reflection. The refractive index of the glass is also a factor 
of importance. According to Harting, if the index is 1.5 about 4 per 
cent of the incident light is lost, while with glass having an index of 
1.6 the loss is about 5.3, so that for a single lens having two glass to 
air surfaces the loss is approximately 10 per cent; for a lens having 
four glass to air surfaces the loss is about 19 per cent, and for one 
having six reflecting surfaces the loss is 26 per cent, while with a lens 
having eight reflecting surfaces more than one third of the incident 
light is lost, or approximately 34 per cent. 

The loss of light at glass to air surfaces was also investigated by R. 
W. Cheshire. He took as a standard a single lens having a relative 
aperture of /’/11 and of 5 millimeters thickness on the axis with the 
following results : ' 


Surfaces y Thickness Y Transmission Equivalent F/value 
Pes See .5 mm. 88.8 F/1r 
Acs cee 2 sa 78.6 F/10.4 
Serato a ah . 69.2 F/9.5 
Sao ae 200. 60.3 F/8.23 

HOLS. sh eee oe cae 52.5 F/7.15 


Thus a single lens with an F/value of F/11 is as fast in practice as 
a four lens system with eight reflecting surfaces working at F/8.23. 

Equally instructive is the comparison made by Dr. Zschokke using 
a Goerz Dagor and Syntor as representative of cemented and air space 
lenses respectively. The results follow in tabular form: 


PHOTOGRAPHIC OPTICS 81 


Syntor Dagor 
Surface 
I sh I r 
re 100.00 94.53 100.00 94.51 
ct as are 93.91 88.77 93.71 93.68 
MR te oii kh, 88.77 84.75 93.48 93:47 
PT ne cn os css os 84.55 80.72 92.65 88.81 
EM we es he. 80.72 77.06 88.81 86.12 
NEE Sa 76.88 73.39 84.37 84.36 
NPOMNOR hs ccs am > 73.39 69.38 84.19 84.16 
OES 68.93 65.16 83.43 78.86 


I = incident light; 1’ — light transmitted. 


In the Syntor, 100 units of light lose at the first surface 5.47 per 
cent, dropping to 94.53 per cent. Absorption causes a further loss and 
reflection at the second surface causes a total drop to 88.77 per cent. 
-The amount lost in the air space is so minute as to be beyond accurate 
measurement and passing on we trace the losses through the remain- 
ing lenses to a final transmission of 65.16 per cent. The last two 
columns show similar figures for the Dagor. In this case there is a 
similar loss at the first surface, but as the difference in the refractive 
index of the cemented glasses is comparatively small there is but little 
loss of light, except at the four glass to air surfaces, and consequently 
the percentage of light transmitted is higher than in the case of the 
Syntor, the actual transmission percentage being 65.16 for the Syntor 
and 78.86 for the Dagor. As both have the same //value, namely 
F/6.8, it will be seen that the Dagor at F'/7.5 has the same efficiency 
as the Syntor at F'/6.8. 

The losses from absorption do not, in general, amount to as much 
as those produced by reflection. Attempts have been made to reduce 
the loss of light by absorption by the use of quartz, which has a much 
greater transparency to ultra-violet than glass. Except for some 
particular purposes efforts along this line have not proved of much 
value. Reduction of the thickness of larger lenses by the use of 
aspheric curves has been tried, but tntil some means is provided to 
enable such curves to be ground more readily, it is not likely that much 
will be accomplished in this direction. 

It is noteworthy that there is also a theoretical foundation for the 
oft asserted fact that the small lenses fitted to miniature cameras are 
faster than larger lenses of the same F'/value. This is due to the fact 
that as the lens becomes larger the thickness of the glass increases, but 
in geometrical ratio, and the loss of light by absorption is correspond- 


82 PHOTOGRAPHY 


ingly increased. The loss from reflection, however, is independent of 
the focal length. 7 

From what has been said, it is evident that the F/number is by no 
means an accurate indication of the speed of the lens, as it does not 
take into account the losses due to absorption and reflection and these, 
as we have seen, are sufficient to cause considerable differences in the 
intensity of the image. Perhaps as time goes on we will find a more 
efficient method based, perhaps, upon the time required to produce a 
definite amount of photochemical action. At any rate, regardless of 
the angle from which the matter is attacked, a method of expressing 
speed which takes into consideration the loss of light due to reflection 
and absorption would certainly be a step in the right direction. 

Variation in the Relative Aperture with Distance of Subject—We 
have seen before that the size of the image varies with the distance of 
the object from the lens. As the size of the image increases as the. 
object is brought nearer the lens, so does the conjugate focal distance, 
v, increase. When the distance of the object is sufficiently great, so 
that the rays of the pencil of light entering the lens are practically 
parallel, the conjugate focal distance, v, is equal to the focal length, | 
f. The intensity of the image is then expressed as d*/f?. However, 
if the object point is brought closer to the lens, the conjugate focal 
distance, v, is no longer equal to f, but becomes progressively greater 
as the object nears the lens. It is evident then that as long as the 
aperture remains constant, the intensity of the image will be repre- 
sented by d?/v? rather than d?/f?. As the conjugate distance varies 
with the distance of the object, it is apparent that the intensity of the 
image, and therefore the time of exposure, will vary with the distance 
of the object point. 

In the vast majority of cases where the object is relatively distant 
from the lens, a condition applying to practically all exterior work 
and embracing the larger number of subjects, the variation in the 
aperture ratio is so small as to be practically insignificant in practical 
work, but in photographing very small objects, copying, enlarging, 
lantern slide reduction and similar work the variation becomes a mat- 
ter of considerable importance and must be taken into consideration 
in calculating the time of exposure. There are two ways in which this 
may be done. The distance from the rear nodal plane to the sensitive 
plate or film may be measured and the F/value calculated from d?/v?, 
but as this method requires a knowledge of the position of the rear 
nodal plane and the effective aperture it is not as convenient as the 


x, i 
: Ly 
a) 
he ee 
‘ on 
ee ee |) 


PHOTOGRAPHIC OPTICS 83 


second method. This consists in basing the exposure on the nominal 
F/value as marked upon the lens and increasing it according to the 
camera extension, v, by multiplying by v?/f?. 

However, as these calculations are usually required only in copying 
and similar work, it becomes more convenient to draw up a table tak- 
ing as a standard the exposure required for copying full size (4 times 
that required with a subject 24 times the distance of the focal length) 
and entering the result of the calculation for relative exposures for 
various extensions by the scale of reduction produced. The follow- 
ing table of the relative exposure for varying proportions of the image 
to the original was calculated by Mr. W. E. Debenham several years 
ago and will be found very convenient in indicating the correction in 
exposure to be made when copying on various scales. Other methods 
of making these calculations are given in the chapter on lantern slides 
and copying. 


Exposures Proportioned 


Proportion of Image to Proportionate to that Required when 
Original (Linear) Exposures Copying Same Size 
1 A ES a ea abt A re oe L.O7 27 
PE 244 0 0 Gh TS aaa 1.10 . .28 
ee es ck ey x svar es L.21 3 
Re iis shee ewes 1.27 31 
ee ay cates 1.36 34 
eh Bg RS ee ee 1.56 -39 
oe) ERE SS A eee ee eee 2.25 .56 
ee ere ks Rive samc 3.06 .76 
eNO SISA) Sc Soa ee hess 4 I 
MI a ort tga. bk id aa 8 a 9 2.25 
Lhe ap he 20 SE Sa a 16 4 
EER ERE es ey Sieg ao bus 25 6.25 
Se ee ee eee 36 9 
Oo Sk A See ne eae 40 12.25 
SRM MET ac eh aa ¥ dy uae sha 64 16 
OO Sain ee eee 81 20.25 
0 he ee a 100 25. 
Of 6 ap “aie, See anne 2 a 121 30.25 
EMC he otra 3 Sveeaa ass sb? 144 36 
OO a SO ESL a a 169 42.25 
OS Gi en a ae 196 49 
Mie en ere AS os uae dean oh 225 56.25 
UN a Po kao uceee 256 64 
RS ea si Ok, oa)a baw «Dees 289 72.24 
RL Bi. 6 Si 'a-u 4% 4,8, sens 324 81 
MM ene 5 5555 pe be seas weet 361 90.25 
NM ln os a ok Goch a hia aneia a wiear 400 100 


84 PHOTOGRAPHY 


GENERAL REFERENCE WORKS 


BecK AND ANDREWs—A Simple Treatise on Photographic Lenses. 
BoLes AND Brown—The Lens. 

CoLte—Photographic Optics. 

Czapsk1—Theorie der Optischen Instrumente. 

Eper—Die Photographischen Objectiv. Ig1o. 

Fapre—Traite Encyclopedique de Photographie. Vol. I and Supplements. 
_ GLEICHEN—Theorie der Modernen Optischen Instrumente. 
Hartinc—Optics for Photographers. 

LuMMER—Contributions to Photographic Optics. 
MetHie—Photographischen Optik. 

MoessArp—L’Objectif Photographique. 

Nuttinc—Outlines of Applied Optics. 

PirpeEr—A First Book of the Lens. 

ScHMipTt—Vortrage tuber photographische Optik. 

SCHROEDER—Die E‘emente der Photographischen Optik. 
STEINHEIL AND Vorr—Handbook of Applied Optics. 
SouTHALL—-Geometrical Optics. 

SouTHALL—Mirrors, Lenses and Prisms. 

TAYLoR—System of Applied Optics. 

Taytor, J. Traitt—Optics of Photography and Phothpr annie Lenses. 
RouHr—Theorie und Geschichte der Photographischen Objectiv. 
WaLLon—Traite Elementaire de L’Objectif Photographique. 
WaLLton—Choix et Usage des Objectifs Photographiques. 


CHAR DER: LY: 
ABERRATIONS OF THE PHOTOGRAPHIC OBJECTIVE 


Introduction.—In a perfect lens image every point in the object will 
be represented by a corresponding point on the flat surface receiving 
the image. It is impossible to realize this ideal and still preserve the 
speed required for photographic purposes because of the defects or 
aberrations to which lenses are subject. The more important of these 
aberrations are: 


Chromatic aberration, 
Spherical aberration; 
Coma, 

Curvature of field, 
Distortion, 

Unequal illumination, 
Astigmatism. 


We will consider at some length the causes, effect on the image and 
manner of correction of each of these. 

Chromatic Aberration.—Chromatic aberration is a defect caused by 
the dispersing properties of glass which prevents a lens from trans- 
mitting white light from a point of the object to a similar point of 
white light in the image. In other words the ray of light on passing 


B 


B 
Fic. 42. ‘Effect of Chromatic Aberration on Definition 


through the lens is broken up into its component colors, the foci of 
which do not coincide but are situated at varying distances along the 
axis as shown in Fig. 42. 

85 


86 PHOTOGRAPHY 


The effect of chromatic aberration on the definition of the lens will 
be evident upon further study of Fig. 42. In this figure A and A’ are 
two rays of white light from an illuminated point of the object. L 
is the lens, uncorrected chromatically, y the point at which the yellow 
rays are brought to a focus, wv the point at which the violet rays are 
brought to a focus and BB’ the sensitive plate. In focussing the 
bright yellow rays are used, since they are the most luminous to the 
eye; the maximum chemical activity, however, lies in the violet and 
blue, so that vw corresponds to. what is known as the chemical or 
actinic focus, y the visual focus. Upon examination of the figure it 
will be observed that when the ground-glass or sensitive plate is at 
BB’ it will receive two images of any point in the object, the one 
formed by the yellow rays coming to a focus at y on the plane surface 
BB’ and the other a disc from the violet rays which having come to 
a focus in front of the plate at v are now diverging, and form on BB’ 
a disk instead of a point. Thus instead of a true point image we have 
the state of affairs shown on the right. If the ground-glass is placed 
at v, the chemical focus, better results are obtained because of the 


Fic. 43. Chromatic Under Correction 


lesser activity of the yellow rays which are beyond the focal plane. 
With color-sensitive plates, however, the disk of confusion becomes 


more prominent. It is in any case sufficient to destroy the clear defini- 


tion of the objective. 


Fic. 44. Chromatic Over Correction 


With a converging or positive lens, chromatic aberration takes the 
form shown in Fig. 43, where v is the focus of the violet and y of the 
yellow rays respectively. This is known as chromatic undercorrection. 

With a diverging or negative lens, we have a different case, which 
is represented in Fig. 44 and is termed chromatic overcorrection. 

The method employed in correcting chromatic aberration will now 


ABERRATIONS OF PHOTOGRAPHIC OBJECTIVE 87 


be evident. When a positive and a negative lens of equal power are 
combined the combination is rendered achromatic. That is to say, 
it transmits white light as such, but on the other hand the two lenses 
being of equal but opposite power neutralize one another so that the 
light rays are not refracted and consequently no image can be formed. 
However, if the positive lens has a slightly higher refractive index 
than the negative lens and the latter a higher dispersion than the 
former, the dispersions may be neutralized without destroying the 
refracting power so that the lens may be at once convergent and 
achromatic. Perhaps a clearer idea of the principle employed may be 
gained with the aid of Fig. 45. In this figure A represents a single 


Fic. 45. Correction of Chromatic Correction 
(From Beck and Andrews, A Simple Treatise on Photographic Lenses) 


convex positive lens with the rays of white light entering on the left. 
Dispersion takes place upon the passage of the ray through the lens 
and the violet rays are brought to a focus at vw and the yellow rays at 
y. In the same figure B represents a negative lens made of the same 
glass and having the same focal power as the above positive lens, but 
of course in the opposite manner. Dispersion again occurs, the violet 
emerging from a virtual focus nearer the lens than the yellow. The 
positions of the foci are exactly the same as those of the positive lens 
because we assume that the lenses are of the same power and made of 


88 PHOTOGRAPHY 


the same glass. If we combine these two lenses it is evident that the 
negative lens will exactly neutralize the chromatic difference of the 
positive lens. However, at the same time the positive character of the 
lens has been neutralized as well and the lens has no longer the prop- 
erty of refracting or focussing light. If for the negative lens we use 
the same type of glass but alter the curves so that it is not as powerful 


as the positive lens we have the condition shown in C of the figure. 


The convex positive lens is only partially neutralized and the chro- 
matic differences persist. However if instead of using a glass of the 
same refractive index we use for our negative lens a glass having 
ereater dispersion we may neutralize the error of the positive lens 
without entirely destroying its refracting power. Thus with a suit- 
able combination we can secure the result of D in Fig. 45, where the 
lens, while not as powerful as the positive lens alone, is free from 
chromatic differences and the various colors are brought to a com- 
mon focus. 

For the sake of simplicity we have been considering only two mre 
violet and yellow, assuming that if these are brought to the same com- 
mon focus the other colors will be brought to the same point. How- 


_ ever, this is not the case and with two pieces of glass it is only pos- 


sible to bring two colors to a common focus. This difficulty is due to 
the fact that the relative dispersions of glass are not the same 
throughout the spectrum. Thus the total dispersion of one glass may 
be twice that of another and lenses from the two glasses may bring 


A Cc DD  E.F G H 


D’ £ Fr G’ H’ 
a ae Irrationality of Dispersion 


any pair of colors together but will not bring the other colors to the 
same focal point unless the dispersion of one glass is double that of 
the other in every part of the spectrum. ~ In most glasses, however, 
the relative dispersion is not constant and the degree of dispersion 
varies with the wave-length of the light as shown in Fig. 46. This is 
known as the “irrationality ” of dispersion. 


a 


ae 


ABERRATIONS OF PHOTOGRAPHIC OBJECTIVE § 89 


The pair of colors chosen for exact coincidence of focus depends 
upon the purposes for which the lens is designed. Thus in instru- 
ments for visual use, as the microscope, the green and orange are 
generally chosen because they, together with the yellow which lies be- 
tween them, are the brightest colors to the eye. However, since the 
plate is more sensitive to the ultra-violet, violet and blue than to light 
of longer wave-length, the colors chosen for photographic purposes are 
the violet, which lies midway between the very active ultra-violet 
and the slightly less active blue, and yellow, which is used for focus- 
sing on account of its luminosity. 

For all ordinary purposes lenses corrected for only two colors are 
satisfactory, since the other colors are either brought very close to the 
common focus or are so much less active that they do not affect the 
sensitive plate sufficiently to destroy the definition. A lens corrected 
for two colors is termed an achromat, or is said to be achromatic. 
The colors which are not brought to an exact focus form what is 
termed the secondary spectrum. 

In three-color process work it is necessary that three colors, instead 
of two, be brought to the same focus in order that the three negatives 
may be equally sharp and of the same size. By the introduction of 
other glasses with the proper calculations it has become possible to 
produce lenses in which three colors are brought to the same focal 
point. These are referred to as apochromats, or are said to be 
apochromatic. ‘These are generally much slower than other lenses 
and are not used to any considerable extent for work other than that 
for which they are designed, since achromatic correction is sufficient 
for all ordinary photographic purposes. 

Spherical Aberration.—Spherical aberration is due to the use of 
spherical surfaces which are the only ones which can be ground 
simply and with sufficient accuracy. It may be described as the in- 
ability of a lens to convey the marginal (not oblique) rays to a point 
at the same distance from the lens as the central rays. In other words, 
the rays passing through the edge of a spherically uncorrected lens 
do not come to a focus at the same point as those which pass through 
near the axis. In spherical aberration we are concerned only with 
the rays parallel to the axis; when oblique rays are considered the 
aberration is known as coma. 

There are two classes of spherical aberration as there are of chro- 
matic aberration. As in the case of chromatic aberration these are 


90 PHOTOGRAPHY 


produced by positive and negative lenses and known as under- and 
overcorrection respectively. 

Fig. 47 represents the condition of spherical undercorrection. ‘The 
parallel rays h,, h,, hz, h, on passing through the lens form image 
points at different distances from the lens, those passing through the 


| 
| 
| 


Fic. 47. Spherical Aberration 


margin of the lens having a shorter focus than those passing through 
the lens at a point nearer the axis. By taking the distances of the rays 
from the axis as ordinates and the distances from the image point for 
the axial zone as abscissee we may construct a curve showing the degree 
of undercorrection present. aft 
Bearing in mind that a negative lens forms only a virtual image, 
parallel rays entering from the right will form virtual image points 
at different distances to the right of the lens as shown in the dotted 
lines of Fig. 47. By using h,, h,, hs, h, as ordinates and the image 


Pek < 


ABERRATIONS OF PHOTOGRAPHIC OBJECTIVE 91 


distances as abscissz, in the same manner as before, a curve can be 


constructed which shows the degree of spherical overcorrection. 


The method of correction consists in compensating the undercor- 
rection of a positive lens by combining it with a negative lens whose 
overcorrection is sufficient to cause the marginal pencils to come to a 
focus at the same point as the axial pencils. It will be remembered 
that chromatic correction was corrected in a similar manner by balanc- 
ing two lenses of opposite errors. To correct for chromatic aberra- 
tion it is necessary that the lenses possess certain definite focal propor- 
tions. Now it is possible to make a lens with a definite focal length in 
many different shapes so that it is possible to take two lenses which 
have the relative focal lengths necessary for chromatic correction and 
at the same time make them of such shapes as will correct spherical 
aberration. ‘Thus since chromatic aberration may be said to depend 
on the focal length of the lenses and spherical aberration on their 
shapes it is possible to correct both of these errors in the same com- 
bination simultaneously. 

In practice it is virtually impossible to completely correct an objec- 
tive for spherical aberration since the curves of over- and undercor- 
rection cannot be made absolutely alike. Fig. 47 shows a lens which 
has the spherical aberration corrected for the center and a zone near 
the margin. The rays h, and h, do not come to the same focus so that 
by taking their distances and the distance of the rays from the axis, 


; “025 +0.25 mm 
Fic. 48. Spherical Aberration in Tessar Ic. (Kellner) 


according to the methods used above, it is possible to construct a curve 
showing the amount of spherical aberration remaining in the combina- 


92 PHOTOGRAPHY 


tion. Perfect correction would be indicated by a straight line but a 
certain amount of uncorrected spherical aberration remains in even 
the very highest grade lenses. Fig. 48 gives an idea of this and shows 
the amount of departure from perfect spherical correction in the 
famous Ic Tessar F/4.5. © 

Coma.—Coma, or zonal aberration, is the name applied to the 
spherical aberration of the oblique rays of light on passing through 
the lens. With spherical aberration proper we are concerned with 
direct pencils of light parallel to the axis and consequently there is 
symmetrical distribution of the light; that is to say, the course of the 
light rays is the same on both sides of the axis. For this reason it 
was only necessary to show one half of the lens in the illustrations of 
spherical over- and undercorrection. 

From Fig. 49 it will be seen that the course of oblique rays on pass- 
ing through a lens is completely unsymmetrical. The rays below the 
axis of the lens are bent more sharply than those above the axis and 
thus do not meet in a common point but in a series of points. Assum- 
ing that the sensitive plate is placed at any one of these points of inter- 
section it is evident that we will not secure an exact image point be- 
cause all of the rays from the corresponding point in the object are 


Fic. 49. Coma. (Kellner) 


not refracted to the same point in the image. In practice instead of a 
sharp, well-defined point we secure a small pear-shaped spot which 
seriously affects critical definition. 

In Fig. 50 a represents the condition known as outward coma, the 
points of the pear-shaped blur facing away from the axis of the lens. 


ABERRATIONS OF PHOTOGRAPHIC OBJECTIVE 98 


If the position of the lens is reversed we have the reverse effect be- 
cause the upper ray is refracted the most and the effect of inward coma 
is produced. In this case, as the name indicates, the points face the 
axis. 

The amount of coma present in any objective may be shown graphi- 
cally by a curve obtained in much the same manner as that which we 


Fic. 50. Two Forms of Coma. (Piper) 


have previously used as an expression for spherical aberration (Fig. 
49). ‘This curve shows the distances of the different points of inter- 
section measured along the axis of the oblique pencil from a plane laid 
through the ideal image point. Perfect correction is indicated by a 
straight line, but as coma is one of the most difficult aberrations to re- 
move, the line is in practice always slightly curved, for while great 
strides have been made in overcoming coma, most modern lenses still 
show measurable amounts. | 

Coma is corrected in two ways: by the use of a diaphragm and by 
compensation. From the illustration it will be seen that a diaphragm 
placed in front of the lens will remove the majority of the oblique 
pencils of light and thus reduce the amount of coma. The principal 
method, however, is by neutralizing the inward coma of one lens with 
an equal amount of outward coma in another lens. If the two lenses 
in Fig. 50 are combined the outward coma of one is neutralized by the 
inward coma of the other and if we assume that the amounts of coma 
present are equal but opposite powers, it is evident that complete 
neutralization will take place and that the pair as a whole will be free 
from coma. Further, since it can be proved that opposite forms of 

8 


94. PHOTOGRAPHY 


coma are given by simply reversing the curvatures of the lens, it is 
possible to find an intermediate form of lens which is practically free 
from coma—a discovery utilized by Mr. H. Dennis Taylor in the well- 
known Cooke triplet objective. 

Curvature of Field.—As the surfaces of the sensitive materials em- 
ployed in general photography are always plane, it is essential that the 
image formed by the lens likewise be plane in order that sharp defini- 
tion be secured over the entire plate. This means that the focus of 
the oblique pencils of light must lie in the same plane as that of the 
axial pencils. With all single lenses, however, the focal points of the 
oblique and axial pencils do not lie on the same common plane but on 
a curve. 7 

With a positive lens (Fig. 51) this curve is concave to the lens 
since the axial pencils come to a focus at a while the focus of an 


Fic. 51. Curvature of Field. (Under Correction) 


oblique pencil is at b rather than at c. When the image curve is con- 
cave to the lens the condition is known as positive curvature of field. 
It may also be referred to as under correction for curvature of field. 


Fic. 52. Curvature of Field. (Over Correction) 


With a negative lens (Fig. 52) the image field is again curved but 
this time the curve is convex to the lens and the condition is then 


vas 


ABERRATIONS OF PHOTOGRAPHIC OBJECTIVE 95 


known as negative curvature of field or sometimes as over correction 
for curvature of field. 

The actual curvature of the image with an uncorrected lens varies 
with the radii, glass, thicknesses of glass, separation of the component 
Jenses and with the position of the diaphragm and the distance.of the 
object. The curvature of field of a positive lens may be removed by 
the introduction of a negative lens if the latter is sufficiently powerful 
and placed at the proper distance. A perfectly flat field is not to be 
expected in any lens, however; least of all in one of the older construc- 
tions, such as the Petzval portrait lens, or the aplanat, where a com- 
promise must be made between curvature of field and astigmatism. 
In the anastigmats the curvature of field is less pronounced, but even 
here all objectives show a slight departure from absolute flatness, but 
the degree of positive or negative curvature is, in the majority of 
cases, not sufficient to cause serious trouble. 

Distortion.—There are several kinds of distortion but the only one 
which we intend to discuss in this place is that due to the inability of 
the objective to reproduce a straight line as such. It is a very objec- 
tionable fault in a number of branches of work such as copying, archi- 
tectural photography and the majority of all scientific work. 

The following diagram will help in explaining without the aid of 
mathematics the general manner in which distortion is produced. Let 
N, and N, (Fig. 53) be the nodal planes of admission and emergence 


Fic. 53. Distortion 


respectively of the lens L, and let BC be a diaphragm placed at some 
distance in front of the lens. The solid lines represent parallel rays 
of light from a distant object passing through the diaphragm, BC, to 
the lens, L, and from thence to a focus at f. Let +N and +N’ be 
parallel lines drawn through the nodal planes of incidence and emer- 
gence. Let d be the point on the image plane where the line N’d 
intersects it. d is therefore the true position for the rays, but owing 


to the fact that a simple lens bends the marginal rays more than the 


96 PHOTOGRAPHY 


central ones, the image point lies not at d, its true position, but at f, a 
point nearer the center. 

When the diaphragm is placed before the lens we have what is 
termed barrel distortion, a state of affairs represented in Fig. 54. 


Fic. 54. Under and Over Correction for Distortion 


When the diaphragm is placed behind the lens the form of distortion 
is reversed and is in this case known as pincushion distortion. It is 
preferable, however, to call the first negative and the second positive 
distortion. 

The method employed in correcting distortion is to combine two 
equal but opposite errors. It has been pointed out that with the 
diaphragm before the lens we have negative distortion, while when the 
diaphragm is placed behind the lens we have positive distortion. Then 
if we use two separate combinations, placing the diaphragm at the 
proper point between the two, the positive error of one will be neutral- 
ized by the negative error of the other and a rectilinear or non-distorted 
image will be produced. 

Unequal Illumination.—With every collecting lens, regardless of 
construction, the center of the field is more strongly illuminated than 
the marginal portions. This falling off in intensity towards the mar- 
gins of the field is known as unequal illumination, or diminution of 
intensity, and is due to two general causes, one of which is regular 
and common to every lens and the other of which varies with the lens 
and is dependent upon the construction of the mount. 


~— ee eee, 


ABERRATIONS OF PHOTOGRAPHIC OBJECTIVE 97 


The regular diminution in intensity is due to three distinct causes: 


1. The constriction of the aperture for oblique rays. 
2. The greater focal length of the marginal rays. 
3. The angling of the marginal rays to the focal plane. 


The manner in which the constriction of the aperture occurs is 
indicated in Fig. 55, where aa and bb represent the limiting rays of 
a direct pencil which can pass through the diaphragm ab. cc and dd 
represent an oblique pencil of light having the same diameter as aa bb. 


Fic. 55. Constriction of Aperture for the Marginal Pencils. (Brown) 


It is easily seen that the whole of this oblique pencil cc dd cannot pass 
through the diaphragm ab because it meets the same at an angle and a 
portion of the pencil of light is cut off by the diaphragm as indicated 
by the shaded portions and by the sections of the aperture A and B. 
Therefore the effective area of the diaphragm is less for an oblique 
pencil than for a direct pencil and consequently the intensity of the 


Fic. 56. Greater Focal Length of Marginal Pencils Resulting in Lower Intensity 
(Brown) 


oblique pencil after passing through the lens is less than the intensity 
of the direct pencil. 
The second point to be considered is the fact that the focus of the 


98 PHOTOGRAPHY 


oblique pencils is at a greater distance from the lens than the central 
pencil. This is shown in Fig. 56, where ab represents the distance of 
the focus for a central light pencil and ac that for an oblique pencil. 
It is evident that ac is greater than ab, or in other words the oblique — 
pencils have further to travel before coming to a focus than the central 
pencils, which again means that their effective value is less. 

Another cause of unequal illumination lies in the angling of the ob- 
lique pencil. The oblique pencil does not strike the plate perpendicu- 
larly, as does the central pencils, but at an angle. Thus in Fig. 57 the 


al b 


Fig. 57. The Angling of the Oblique Ray. (Brown) 


surface on which it would fall perpendicularly is RS, which is at the 
angle cSR to the sensitive plate cb. The area of each image point, © 
represented by ES, becomes cS and the intensity from this cause is 
therefore less than that of the central pencils as SE : cS. 

The amount of the reduction in the intensity of the image at any 
point removed from the axis due to the above causes may be calculated 
mathematically provided there is no obstruction of the oblique rays by 
the lens mount to be considered. Formulz for calculating the diminu- 
tion in the intensity of the lens image at various distances from the 
axis were given by R. H. Bow as early as 1866.1 This relation is rep- 
resented in Fig. 58; ? the angles of view subtended by the diagonal of 
the plate are marked along the top of the graph, while the numbers 
below are the ratios of the diagonal of the plate to the focal length of 
lens corresponding to the angle of view marked on the top line above 
the graph. The vertical line is marked in exposures, or intensity 
units, starting with a unit intensity of one on the base. 

The other cause of unequal illumination les in the obstruction of 
the oblique pencils by projecting lens mounts. This occurs whenever 
the aperture of the lens is very large in proportion to the length of the 


1 Brit. J. Phot., 1866, p. 160. 
2 Zschokke, Brit. J. Phot., 1917, 64, 203. 


ABERRATIONS OF PHOTOGRAPHIC OBJECTIVE 99 


mount itself. Most modern lenses, particularly anastigmats, are very 
compactly built with their components close to the diaphragm and the 
illumination is consequently more uniform than in the case of older 


uP LY 33° 44° SS° 62° 70° 717° BH 90° 95° 100° 405° 409° 


Redes isls)caloof | [a] 1) 
eee 
SERRE ee 

isis ab pe 


ao +- Ww 


Exposure at corn 


nN 


2 = G6 (8 40 42 44 146 18 20 22 24 26 28 
Diagonal divided by focal Length 


Fic. 58. Relation between the Angle of View and the Diminution of the 
Optical Intensity of the Image. (Zschokke) 


lenses, such as the Petzval portrait objective, which have a much larger 
distance between the components in proportion to the relative aperture 
than do modern anastigmats. 

Astigmatism.—Astigmatism is one of the most serious of the aber- 
rations and is at the same time one of the most difficult to correct. 
While it is not strictly accurate to say that an astigmatically corrected 
objective was possible only after the introduction of the newer varieties 
of glass following the investigations of Abbe and Schott, since Martin 
as well as Beck ® have shown that anastigmatic objectives may be con- 
structed without these glasses, the products of the Jena glass works 
have played an important part in the development of the anastigmatic 
objective, the series of barium crowns being particularly notable for 
having contributed largely to the rapid development in objectives of 
this type. The anastigmatic objective may be said to date from the 
introduction of the Protar by Rudolph in 1890.* 

Astigmatism is an error which affects only those light pencils which 
pass through the lens obliquely. It is due to the lens converging the 
oblique pencils of light to two separate focal lines rather than a point. 
Astigmatism differs from spherical aberration in that the latter affects 
the central as well as the marginal definition, while pure astigmatism 

8 Martin in the Omnar produced by Emil Busch, Beck in the Neostigmar 
Series. 

4D. R. P. 56,109—April 1890. 


100 PHOTOGRAPHY 


is an error found only on points removed from the axis. Spherical 
aberration of the oblique pencils may also exist and has already been 
discussed under Coma. 

When a pencil of light falls obliquely on the surfaces of the refrac- 
tive medium the course of the rays in the different planes becomes dis- 
similar and we must distinguish between two special planes. One of 
these is the plane which passes through the principal ray of the oblique 
pencil and is represented as the plane of the drawing. This is termed 
the meridional plane. Perpendicular to this is the equatorial plane. 

The condition of astigmatic deformation is shown in Fig. 59.2 The 
pencil of the light from the window bars at the center of the field passes 
along the axis and hence the image at the focal point is an exact point 
for point image, chromatic aberration being assumed to be absent. 


= 
D 
——S 
—— Fs 
38. 


or oe 


Les 

Cas 
es 

Ss 


WINDOW Bans AT Manan oF Fe 
LD 


Fic. 59. Astigmatic Deformation. (Kellner) 


The pencil from the window bars on the margin of the field, however, 
passes through the lens obliquely and in so doing the two planes be- 
come unequally refracted and come to a focus at different points. 
Consequently we do not secure a perfect image of those points which — 
lie removed from the axis but instead we have a series of image points. 
Thus the vertical bar comes to a focus (t) before the horizontal bar 
and when the former is sharp the latter is not. If the position of the 


5 Courtesy of Dr. Hermann Kellner and the Society of Motion Picture Engi- 
neers, 


ABERRATIONS OF PHOTOGRAPHIC OBJECTIVE 101 


ground glass is altered in the direction of s the horizontal bar is brought 
to a focus while the image of the vertical bar becomes less and less 
sharp. Hence it is impossible to secure a sharp image of both at the 
same time no matter in what position the ground-glass is placed. The 
distance between the focus of the rays in the meridional and those in 
the equatorial plane forms what is termed the astigmatic difference. 

From the sectional illustration representing the appearance of the 
cross bar when the ground-glass occupies various positions between 
the astigmatic image points ¢ and s, it will be seen that there is a point 
where both lines are equally sharp although neither is critically sharp. 
This point represents what is termed the circle of least confusion. If 
the image point lies near the axis, or the outstanding error is small, 
the diameter of the circle of least confusion may be so small as to be 
for all practical purposes a point image. ‘The lens is then considered 
to be astigmatically corrected and is termed an anastigmat. 


peek 30 


25 


i) 

20 20 

H 

! 

5 15 

10 10 

5 

ae 0 -3 O+1 -1 0 +3 -4 0 -8 : O+1 


Fic. 60. Astigmatic Curves. (Von Rohr) 


By swinging the window bars nearer the axis the astigmatic differ- 
ence becomes less and less until finally when the bars reach the axis 
astigmatism disappears. Taking the different angles which the princi- 
pal ray may take with the axis, we obtain a series of astigmatic image 
points which when connected give two astigmatic curves. The char- 
acter and extent of these two curves afford a means of illustrating 
graphically the amount and character of the astigmatism present in the 
lens, The general shape of these curves is shown in Fig. 60, the dotted 


102 PHOTOGRAPHY 


line representing the image points of the meridional rays and the solid 
line those of the equatorial rays. Occasionally, however, the astig- 
matic curves take different shapes as shown in a, b and c of the same 
figure. Where the two curves coincide we have astigmatic correction. 
The deviations represent what are termed astigmatic zones or zonal 
errors. ‘The amount of these errors is dependent upon the aperture 
and the construction of the objective and it is the aim of the designer 
to remove or to reduce these as much as is possible under the condi- 
tions. | 

Unfortunately, however, the difficulties of the designer do not end 
here. Generally when the astigmatic zones have been removed and 
astigmatic correction secured the image points lie on a curve and not 
ona plane perpendicular to the axis of the objective. Further calcula- 
tion is then necessary in order to bring all of the astigmatic image 
points as close to a plane surface as possible. When this is accom- 
plished we have what is termed an anastigmatically flattened field. 

Flare and Flare Spot.—Both flare and flare spot can hardly be 
termed aberrations as they are not concerned with the formation of 
the primary image, but as they are properties of lenses which affect 
the character of its image it seems well to treat them at this point. 

There are two kinds of flare, one caused by reflection of light from 
a bright object within the lens mount and generally termed flare spot 
and that due to reflection of light from the surfaces of the lenses 
themselves. The former may be termed mechanical flare and the lat- 
ter optical flare. The first can be avoided and there is little danger 
of flare from this source with a lens of a reputable manufacturer, 
unless old or damaged. Second-hand lenses should be carefully ex- 
amined for unblackened spots on the mount before purchasing, while 
the same cause may be looked for when an old lens suddenly begins to 
give flat foggy images. 

Optical flare cannot be avoided completely in any lens, and a lens 
may be excellently corrected otherwise but still useless on account of 
strong flare. Fig. 61 will illustrate the manner in which optical flare 
is produced. Let a and a’ represent two parallel rays of light passing 
through the diaphragm and then through the lens and coming to a 
focus at b. There is a certain amount of reflection at each surface 
and in the case of the second surface the light is reflected back to the 
first surface, where it is again reflected back and reaches the plate at 
cand c’. If the focus of the secondary reflected image is near the 


ABERRATIONS OF PHOTOGRAPHIC OBJECTIVE 103 


same plane as the lens image, a definite spot is formed which destroys 
critical definition and gives a hazy, foggy effect. Any increase in the 
number of glasses in the objective increases the number of reflecting 
surfaces and hence the greater danger from flare in the more complex 
forms of modern lenses than in the old single achromat. In addition 
_ the deeper the curves of the individual glasses the greater the per- 
centage of light reflected and consequently the greater the danger of 


2 
are eae b 
a aa a 


Fic. 61. Optical Flare 


flare. The presence of air spaces increases the number of reflecting 
surfaces so that of two lenses of the same glasses and of the same 
design the amount of flare will be greater theoretically in the lens in 
which the components are separated by air spaces. 

Some forms of modern anastigmats are more subject to flare than 
others and with all it is advisable to take all possible precautions to 
remove all sources of the same. Much can be done by the habitual use 
of an efficient lens hood which really ought to be regarded as an in- 
tegral part of every ultra rapid objective. 


GENERAL REFERENCE WorKS 


The various aberrations of photographic objectives are considered in practi- 
cally all of the reference works given in the bibliography following chapter ITI. 
The following may be recommended as being especially suitable. 


Hartinc—Optics for Photographers. (English translation by Fraprie.) 1912. 
LuMMER—Photographic Optics. (English translation by Thompson.) 1903. 
Von Rour—Theorie und Geschichte des Photographischen Objectivs. 1899. 


CHAPTER V 


THE PHOTOGRAPHIC OBJECTIVE 


Introduction.—This chapter is a brief survey of the common types 
of lenses and the principles employed in their construction. The num- 
ber of lenses which differ but slightly from a few well-established con- 


structions is almost without number and owing to the limitations of — 


space it is impossible to treat all of these. Accordingly the chapter 


will be devoted to the more important principles of construction which ~ 


have been widely copied on account of their admitted excellence. 

For a complete history of the development of the photographic ob- 
jective Von Rohr’s monumental work, Theorie und Geschichte des 
Photographischen Objektivs, should be consulted. Although published 
in 1899 and therefore antedating many of the anastigmats this work 
still holds its value as the most complete history of the photographic 
objective in any language. Eder’s Die Photographischen Objektive 
is also a very valuable source of reference and is more complete as 
regards the later lenses. A less comprehensive survey, yet one which 
is perhaps of more real value to students, is to be found in Optics for 
Photographers, by Hans Harting; while the chapter on the photo- 
graphic objective in the English translation by McElwain and Swain 
of Gleichen’s Theory of Modern Optical Instruments is of value. 


Part I. THe ASTIGMATS 


Single Lenses.—It is impossible to correct a single lens in any way, — 
hence it is unable to give sharp definition except with a very small 
diaphragm with the consequent sacrifice of speed. The spherical 
aberration is at the minimum when the lens is double convex and the 
radii of the surfaces are in the proportion of 1:6. Such a lens, how- 
ever, is useless photographically because it fails to cover a flat field 
satisfactorily. Even with small stops the image is sharp only in the 
center of the field and rapidly falls off towards the margins. In 1812 — 
Wollaston showed that a much better image could be obtained witha — 
concavo-convex, or meniscus, lens than with the usual bi-convex:. 
Wollaston’s meniscus (Fig. 62) with the concave side towards the sub- — 
ject gives an image of satisfactory sharpness over a limited field when 

104 


THE PHOTOGRAPHIC OBJECTIVE 105 


medium-sized stops are used. Since chromatic abetration cannot be 
corrected in a lens composed of a single piece of glass, the visual and 
chemical foci do not coincide and a correction must be made after 
focussing. The amount of this correction is equal to the focal length 
of the lens divided by the v constant of the glass of which it is com- 
posed. With the glass generally used for lenses of this type the differ- 


Fic. 62. Wollaston’s Meniscus 


ence of the two foci, or the chromatic difference, is equal to about 2 
per cent of the focal length. The principal use of lenses of this type 
now is for diffused focus, impressionistic photography. 

Single Achromatic, Lenses.—In the preceding chapter it was shown 
how it is possible to correct both chromatic and spherical aberration at 


the same time by cementing to a single collecting lens a dispersing lens 
of the proper power. A lens so constructed is termed an achromat, 


or is said to be achromatic, i.e. chromatically corrected. Such lenses 
may be comparatively well corrected spherically and are able to give 


sharp definition over a field of medium extent when used at a maxi- 
mum opening of about F/14 to F/16. 


\\\\\ 
\ 
YY 


\\ 


A 


, 


XN 
il 


Up 
wy 


AAT AAA 


AKIN 


AN 
LU 


\\ 


mig, 02, Lhe Chevalier or French 


Fig. 64. 
Landscape Lens 


Grubb’s Landscape Lens 
Many lenses of this type were constructed in the early days of 
photography. (See Von Rohr’s work for a complete account.) As 
an example of a single achromat composed of a double concave negative 
lens cemented to a double convex collecting lens we may mention the 


106 PHOTOGRAPHS 


lenses of Chevalier of Paris, Francais of Paris, Busch of Rathenow, 
Goerz of Berlin, and Voightlander of Brunswick. In practically all 
of these (Fig. 63) the positive lens is of crown and the negative lens 
of flint. 

In 1857 Grubb patented an achromatic lens composed of two con- 
cavo-convex meniscus lenses cemented together, the glass nearer the 
diaphragm being of crown and the other of flint (Fig. 64). In 1864 
Dallmeyer introduced his ‘‘ Rapid Landscape Lens ” which is similar 
to the above (Fig. 65) but differs in the introduction of a third me- 


\ Wy, 


— 


_—<s 


Fic. 65. Dallmeyer’s Rapid Landscape Lens 


niscus of crown glass for the purpose of securing better correction. 

The “ single wide-angle Landscape Lens” of Ross, the “ Amateur- 

linse”’ of Goerz, and a large number of other single landscape are of 
similar construction. 


ee ee 


Ye 
NSS 


MN Yf / . 
TKS XK 
QS = S 
we LEO LIIRISIITT TE, le RIAL TSL ELL 


WiLL 


WY 
SS \ 


Yf 


Fic. 66. Goddard’s Landscape Lens—Dallmeyer’s Rectilinear Landscape Lens. 


In 1869 Goddard described a lens having the form shown in a of 
Fig. 66. For the purpose of securing better correction a convexo- 
concave meniscus of flint was added to the usual combination of crown 
and flint, an air space separating the meniscus from the double-con- 
eave negative lens. In the Dallmeyer “ Rectilinear Landscape Lens” — 
patented by Dallmeyer in 1888 (see Photographic News, 1889, p. 59) _ 


Pere HOLOGRAPHIC OBJECTIVE 107 


_the same principle is followed, the two cemented lenses being of crown 


glass and the separated meniscus of flint. This construction repre- 
sents, perhaps, the highest attainment of a lens of this class. It has a 
relative aperture of //14 and produces an image of satisfactory sharp- 
ness over a comparatively large field. The author has used one of 
these lenses for certain kinds of work for years and has found it per- 
fectly satisfactory. 

Semi-achromatic Objectives or the Anachromats.—The use of 
partially corrected lenses at a large aperture so as to secure diffused 
images may be said to date from the introduction of such lenses by 
the French pictorialist, M. Puyo, and the construction of the Dall- 
meyer Bergheim for Mr. Bergheim, a painter, in 1896. 

All of the lenses within this class are only partially corrected for 
chromatic or for spherical aberration and to this they owe the peculiar 
diffusion or “ enveloping image ” expressed so admirably by the French 
word “ flou.” The Struss Pictorial Lens and the Kalosat are concavo- 
convex meniscus lenses of the type represented by Wollaston’s me- 
niscus. The Smith Semi-achromatic, and Synthetic, the Spencer Port- 


land, the Gundach Single Achromatic, Bausch and Lomb Plastigmat, 


Degen Objectif Anachromatique, Hermagis Eidoscope, Koristka Ars 
and Dallmeyer Sofi-Focus are all lenses resembling the single achro- 
matic lenses previously mentioned in construction, but differing from 
them in the use of a much larger aperture and less thorough correction 
spherically. The majority of these lenses are chromatically corrected 
although the chromatic aberration of some is only partially carried out. 

The Gundlach Hyperion, Wollensak Verito and Smith Visual-Qual- 
ity are double lenses formed of two components which alone are es- 
sentially single achromatic lenses. In the Hyperion three foci are 
available as both components may be used separately. In the Verito 
two foci only are available as only the back component may be used 
alone while the components of the Smith Visual-Quality cannot be used 
separately. The Dallmeyer-Bergheim and the Hermagis Teleidoscope 
are semi-achromatic lenses constructed on the telephoto principle. The 
former consists of a single positive and a negative lens, the space be- 
tween the two being adjustable by rack and pinion. Varying the dis- 
tance between the two elements alters the focal length so that the 
single objective is equal to a battery of lenses having a fixed focal 
length. The lens is not chromatically corrected and a correction must 
be made after focussing. The maximum aperture is F/6.5. The 


_Hermagis Teleidoscope is similar in construction but has a relative 
aperture of F/6 and is chromatically corrected. 


te ee 


108 PHOTOGRAPHY 


Many of the longer foci anastigmats, which are used principally for 
portrait work, and also some of the Petzval type portrait lenses are 
fitted with diffusing devices which introduce a certain amount of aber- 
ration and soften the critical sharpness of the fully corrected image. 

The Double Achromat-Aplanat or Rapid Rectilinear.—The single 
objectives all possess, in addition to astigmatic deformation, coma, 
curvature of field and incomplete spherical correction, the very serious 
defect known as distortion. That is, straight lines cannot be repro- 
duced as such but are rendered as parts of curves and as the diaphragm 
must be placed before the lens with a single objective, the distortion is 
barrel shaped. By constructing a symmetrical objective consisting of 
two achromats the diaphragm may be inserted between the two so that 


“H}- Siae 


Fic. 67. Sutton’s Triplet Fic. 68. Dallmeyer’s Triple Achromatic 


the barrel distortion of one element is balanced by the pincushion dis- 
tortion of the other and the fault entirely corrected. In addition, 
owing to the superior correction which may be effected with the sym- 
metrical construction, a larger working aperture, and consequently 
greater speed, is secured. 

Thomas Ross appears to have been the first to make symmetrical ob- 


Fic. 69. Harrison and Schnitzer’s Globe Lens 
jectives. Ross, however, does not seem to have realized the advan- 
tages of the symmetrical construction in eliminating distortion and for 
reducing the amount of aberration so that a larger aperture may be 
obtained. 


THE PHOTOGRAPHIC OBJECTIVE 109 


In 1858, Thomas Sutton described a symmetrical triplet objective 
composed of a pair of cemented achromatic lenses with a double-con- 
cave negative lens between (Fig. 67) and in 1860 Dallmeyer intro- 
duced a lens of similar construction (Fig. 68) in which all three ele- 
ments were cemented achromats. The same year Harrison and 
Schnitzer brought out the “globe lens” (B. P. 2496/1860). This 
consisted of two cemented achromatic elements and was termed the 
globe lens because the inner and outer surfaces both formed portions 
of spheres having a common center (Fig. 69). It was free from dis- 
tortion but as it was not well corrected spherically it had to be used 
with a comparatively small stop. It was therefore soon replaced by 
the aplanat and rapid rectilinear of Steinheil and Dallmeyer respec- 
tively. 

A great advance was made in 1866 when Steinheil of Munchen in- 
troduced a symmetrical objective which he termed the aplanat (Fig. 
70). About the same time, or shortly thereafter, J. H. Dallmeyer of 


Fic. 70. The Aplanat or R. R. 


London independently discovered the same construction which he 
patented under British Patent 1641 and 2502 of 1866. Steinheil evi- 
dently reached the conclusion that astigmatism would be lessened if the 
refractive indices of the two glasses employed in constructing the 
single achromats of a symmetrical objective were more nearly equal. 
Therefore instead of employing flint and crown glasses, as had his 
predecessors, he used instead two flint glasses. Dallmeyer also used 
two flints. 

The first of Steinheil’s aplanats had a relative aperture of F/8; 
while an even more rapid objective designed for portrait work was 
patented at a later date. The aplanat is made in several series having 
various speeds according to the requirements of the work for which 

9 


110 , PHOTOGRAPHY 


they are intended. Roughly these series may be termed the universal 
aplanat, the group aplanat, the portrait aplanat, and the wide-angle 
aplanat. 

It is the universal aplanat with which the public is most familiar. 
This has an aperture of about F/8 and in many cases allows separate 
use of the components at F/14 to F/16. At these apertures the defini- 
tion is satisfactory for all but the most critical work or where the re- 
sults must be subsequently enlarged. The correction for chromatic 
aberration and distortion is very good but there is a small amount of 
spherical aberration, curvature of field and astigmatism remaining. 
However, where high speed is not required and the lens may be safely 
stopped down to a small opening, the rapid rectilinear is perfectly 
satisfactory and is an objective which does not involve a large outlay. 
The various makes are all made according to practically the same 
formula and there is no advantage in our discussing the various lenses 
individually. 

The group or portrait aplanats are not so well corrected as the uni- 
versal aplanat but they are sufficiently corrected for the purpose for 
which they are intended. The apertures of lenses of this class range 
from F/6 to F/4. The Meyer Series B Aristoscope F/5.5, Berthiot’s 
Eurygraphes Symetriques F/6, Suter’s Rapid-Aplanat F/5, Voigt- 
lander’s Euryscope F/4, Goerz Paraplanate F/5.5, Gundlach Recti- — 
graphic F/5.6, Wollensak Versar F/6 as well as many other lenses no 
longer made fall into this class. 

Practically all wide-angle lenses which are not anastigmats are of 
the aplanat construction. For this purpose curvature of field requires 
to be kept as low as possible while coma and spherical aberration 
must be highly corrected: therefore the average wide-angle aplanat 
has a small maximum opening, generally about F/16 to F/18. 

The Petzval Portrait Lens.—The construction of the Petzval por- 
trait lens by Voigtlander in 1840 from calculations by Joseph Petzval, 
a mathematician of Vienna, may be said to mark the beginning of the 
serious designing of photographic objectives, as well as the beginning 
of portrait photography. Before this time the photographer had at 
his disposal only the single lens with a small aperture and with the in- 
sensitive materials then available the exposures were so long that por- 
traiture was practically out of question, except under the most favor- 
able conditions. The Petzval portrait lens however changed all this 
and with its aperture of F/6, which was almost immediately increased 
to F/4 by Andrew Ross, portraiture for the first time became really 


Qa PHOTOGRAPHIC OBJECTIVE 111 


practicable. The development of photography owes much to, Petzval’s 
achievement, for coming directly after the discovery of the daguerre- 


Fic. 71. Portrait of Petzval 


otype it made that process of practical value and thus immensely in- 
creased the importance of the subject to the general public. 


Fic. 72. Petzval’s Portrait Objective 


The Petzval lens consists of four lenses in two combinations (Fig. 
72). The front combination consists of a positive lens of crown glass 
cemented to a negative lens made of flint glass, while the rear combina- 
tion consists of two separate lenses, the negative lens being convexo- 
concave and of flint, while the positive lens is double-convex and made 


112 PHOTOGRAPHY 


of crown. It is completely unsymmetrical and, because of its good 
correction for spherical aberration and coma, the central sharpness is 
excellent. It is free from distortion and is chromatically corrected. 
In opposition to these valuable features it has faults which limit its 
usefulness and have caused it to be practically replaced by the later 
anastigmats, except for studio portraiture. It is not astigmatically 
corrected, it covers a very small field, has a decided curvature of field, 
and owing to its length there is a considerable amount of vignetting, 
or a diminution of intensity towards the margins, which renders it un- 
suitable for landscape or other work requiring sharp definition from 
corner to corner of the plate. Nevertheless, appearing as it did before 
any serious attention had been paid to the subject of photographic 


Fic. 73. Modifications of the Petzval Portrait Objective 
((a) Dallmeyer, (b) Voigtlander, (c) Zinc-Sommer) 


lens designing, it marks a most brilliant achievement—the greatest 
single achievement in the annals of the photographic objective. 

Petzval’s original construction has been several times modified by 
later opticians in order to improve its performance. The most im- 
portant changes are those of Dallmeyer, Voigtlander and Zince-Som- 
mer, 


THE PHOTOGRAPHIC OBJECTIVE 118 


In 1866 J. H. Dallmeyer modified the original Petzval design so as 
to obtain better spherical correction. The change consisted in revers- 
ing the glasses of the rear combination, placing the flint glass in the 
rear, the lens in other respects remaining practically unaltered (Fig. 
73)- : 

In 1879 (D. R. P. 5761/1879) Voigtlander changed the back com- 
bination to a plano-convex collecting lens of crown to which he ce- 
mented a concavo-convex lens of flint, the air space being removed 
completely (Fig. 73). 

In 1870 H. Zincke Sommer further modified the original design in 
order to obtain an increase in the relative aperture. The change con- 
sisted in placing the positive lens before the negative and leaving an 
air space between as shown in Fig. 73: ‘The relative aperture was by 
this means increased to F'/2.3. 

Practically all of the modern portrait lenses, which are not anastig- 
matic, are constructed according to the Petzval formula, or on calcu- 
lations based upon the same, and it is beyond the scope of this work 
to discuss the various lenses of this class now on the market and little 
of practical value would be gained by so doing. 


PART Il. THE ANASTIGMATS 


Introduction.—Regardless of how well they may be corrected other- 
wise all the lenses which we have hitherto investigated contain a seri- 
ous amount of astigmatism. The character, cause, and correction of 
astigmatism were discussed in the preceding chapter and if the mat- 
ter is not clearly in mind this section of the chapter should be re- 
viewed before proceeding. 

Steinheil made one step towards the correction of astigmatism in 
the construction of the aplanat but complete correction was possible 
only after the production of the newer varieties of glass by the Jena 
Glass Works. In order to correct spherical aberration in a cemented 
system it must have a surface convex to the medium of higher re- 
fraction, while in order to correct astigmatism the surface must be 
convex to the medium of lower refractive index. With the varieties 
of glass known before the introduction of Jena glass the refracting 
power increased in the same ratio as the dispersing power and it was 
impossible to make a spherically corrected achromat that would also 
be anastigmatic. 

However after the introduction of Jena glass it became possible 


114 PHOTOGRAPHY 


to make the collecting lens of glass having higher refraction and lower 
dispersion than the dispersing lens and thus secure astigmatic correc- 
tion. The old achromat made of ordinary crown and flint could be 
corrected spherically but not astigmatically ; the new achromat, how- 
ever, while corrected astigmatically cannot be spherically corrected. 
However, by combining the ordinary crown and flint of the old achro- 
mat with the barium crown glass of the new achromat it became pos- 
sible to correct both astigmatism and spherical aberration in the same 


lens. By combining two such achromats both of which are individ- . 


ually corrected to form a symmetrical objective a larger aperture be- 
comes possible together with correction for curvature of field, coma, 
and distortion. This is the principle followed in the construction of 
most of the symmetrical lenses as the Goerz Double Anastigmat or 
Dagor, the Voigtlander Collinear, Turner-Reich Anastigmat, etc. 
The other method is to use two dissimilar combinations, one of which 
is spherically corrected while the other is astigmatically corrected, 
the two when placed on opposite sides of the diaphragm forming an 
unsymmetrical objective which is completely corrected as a whole but 
not individually. This is the principle followed in the construction 
of most of the high-speed anastigmats such as the Tessar, Heliar, etc. 

Cemented Symmetrical Objectives.—The Double Anastigmat—The 
Goerz Double Anastigmat, better known in this country as the Dagor, 
is a symmetrical objective consisting of two similar combinations each 
of which is composed of three lenses (Fig. 74). The indices of re- 


Fic. 74. The Goerz Dagor 


fraction of the three glasses increase as we pass from one glass to 
another in the direction of the incident light, ie. from the diaphragm, 
as we are considering the single element. Hence the first cemented 
surface is convex to the medium of lower refraction satisfying the 
requirements for spherical correction. The second cemented surface 
is convex to the medium of higher refraction and thus satisfies the 
requirement for astigmatic correction. Consequently the single ele- 
ment of three cemented lenses is corrected both spherically and astig- 
matically in a very simple manner. 


= fn 
ee ee ee : 


Pay 


THE PHOTOGRAPHIC OBJECTIVE 115 


The Dagor is excellently corrected. Its single elements do not 
equal the best corrected single lenses, but at an aperture of F/13 give 
a degree of definition that is satisfactory for most purposes. The 
relative aperture of the symmetrical objective is F/6.8 for the shorter 
foci and somewhat less for the longer. Particularly noticeable is the 
wide field which is critically defined at large apertures and also the 
extension of this field by the use of smaller stops. In this respect the 
Dagor is hardly to be surpassed by any other objective. 

Among other lenses which are constructed according to practically 
the same principles may be mentioned the following: 


Meyer Double Aristostigmat, F/6.8, F/5.4, F/4.2, 
Plaubel Triple Orthar, F/6.8 and F/5.4, 

Koristka Meridan, F/6.8, 

Hermagis Aplanastigmat, F'/6.8, 

Zeiss Amatar, F/6.8, 

Degen Double Anastigmat “ Normos,” F/6.3, F/7.4, 
Wray Universal Anastigmat, F'/6.8. 


Alternate Form of the Double Anastigmat.—The principle em- 
bodied in the design of the Double Anastigmat just described is sub- 
ject to several alterations. A second arrangement consists in the use 


Fic. 75. Watson’s Holostigmat 


of two negative lenses, one having a lower, the other a higher index 
of refraction than the interposed positive lens (Fig. 75). Here the 
arrangement of cemented surfaces is reversed, the indices of re- 
fraction of the three glasses decreasing as we pass away from the 
diaphragm. This form of the double anastigmat has been made for 
several years by Watson of London as the Holostigmat in two series, 
one with a maximum aperture of F/4.6 and the other F/6.1. The 
Steinheil Satz-anastigmat also has this design. Practically there is 
little difference in this form and the one previously described and the 
corrections of both may be equally well carried out. 

A further modification was invented by two men, Kaempfer of 


116 . PHOTOGRAPHY 


Brunswick and Steinheil of Munich, independently and introduced 
by Voigtlander as the Collinear and by Steinheil as the Orthostigmat. 
Two forms are described in Kaempfer’s patent specification. 

The second form (Fig. 76), which was adopted in the manufacture 
of both the Collinear and the Steinheil Orthostigmat, consists of a 


meniscus of low refraction interposed between a double-convex 


positive lens of high refraction and a double-concave negative lens 
of medium refraction. If the central meniscus having low refraction 


Fic. 76. The Collinear and Orthostigmat of Voigtlander and Steinheil 


is made quite thick the correction can be carried out very well so that 


the single elements may be used separately 1f stopped down. The 
second form is superior to the first in that it can be more easily cor- 
rected for astigmatism while the first design is more easily corrected 
spherically. A combination of the two using one combination of each 
design for a double objective was proposed by the inventor but never 
adopted. The relative aperture of the Collinear of Voigtlander is 
F/5.6 while the Steinheil Orthostigmat is made in series up to a 
maximum aperture of F/6.8. ; 

The Four Glass Element—the Protars.—Rudolph’s earlier Protars 
were unsymmetrical anastigmats consisting of two dissimilar com- 
binations, one being an old achromat spherically corrected and the 
other a new achromat astigmatically corrected. With the exception 
of a series for wide-angle work these are no longer made. The series 
VIIa Protar with which we are most familiar is a symmetrical anastig- 
mat containing two like combinations each of which consists of an 
old and a new achromat (Fig. 77). The single element thus consists 
of four cemented lenses, the two nearest the diaphragm comprising 
the new astigmatically corrected achromat, while the other pair-are 
similar to the old Steinheil aplanat and form the spherically corrected 
old achromat. 

This single element is the basis of the convertible Protar Series 
Vila. The speed of the single elements is F/12.5 and because of the 


a ee 


ry 
* sy 
- 
ee 


THE PHOTOGRAPHIC OBJECTIVE 117 


increase in the number of glasses to four the lens is excellently cor- 
rected, its nearest competitors as regards definition being the single 
elements of the three glass type previously described. When a single 
element is used separately it should always be placed behind the 


Fig. 77. Rudolph’s Protar Series VIIa 


diaphragm, as it is corrected for rays incident in that direction. When 
the complete double objective is used the element having the longer 
focal length should be placed in front, as the correction of the double 
objective depends on having the course of the light rays between the 
two elements parallel. In comparison with objectives formed of 
three-lens elements, the single element of the four-lens element is 
superior but the same does not necessarily hold for the double ob- 
jective. 

The design of the Ross Combinable is identical but by changes in 
the glasses used it has been possible to increase the aperture of the 
single element to F/11 and the double objective to F/5.5 without sac- 
tificing in any way the corrections of the objective. The principal 
change consists in the use for the double-concave negative lens, of 
fluor-crown glass having very low refraction and dispersion for the 
boro-silicate crown glass usually used. 

The Five Glass Element.—Before the introduction of the anastig- 
mat a form of the aplanat was made consisting of three instead of two 
lenses for the purpose of securing better correction. When the in- 
troduction of the newer glasses made possible the construction of the 
new astigmatically corrected achromat Turner and Reich added the 
new achromat to their former three glass aplanat to form the Turner- 
Reich anastigmat (Fig. 78). This has an aperture of F/6.8 and is 
distinguished by the excellent correction of its single elements. Espe- 
cially noticeable is the great covering power of the lens at full aper- 


118 é PHOTOGRAPHY 


ture, which is greatly increased if a smaller step is used, and the lens 
may then be used as a wide-angle lens on a plate much larger than 
that for which it was originally designed. In this respect the Turner- 
Reich anastigmat is unsurpassed. 


Fic. 78. The T-R Anastigmat 


r, 
| 


Symmetrical Lenses with Air Spaces—Celor and Syntor of Goerz. 
—In a cemented system of three lenses containing a bi-concave and 
a bi-convex lens with an interposed collecting meniscus of low refrac- | 
tion, such as the Steinheil Orthostigmat, anastigmatic flatness is se- 
cured and spherical aberration corrected by the use of two spherical ce- 
mented surfaces, one of which acts as a collecting and the other as a 
dispersing lens. Inthe Celor and Syntor of Von Hoegh (Fig. 79) the 


Fic. 79. The Celor and Syntor of Goerz 


interposed collecting meniscus is replaced by an air space, so that it 

consists essentially of a double-convex collecting lens separated from 7 
a double-concave negative lens by an air space having the form of | 
a positive meniscus. Corresponding to the three-lens system this two- 

lens system contains two contact surfaces, one of which is collecting 
and the other dispersive, but the contact surfaces are between air and | 
not glass of low refraction. The two-lens system may thus be re- 
garded as a system derived from the three-lens element by decreasing 


pre PHOTOGRAPHIC OBJECTIVE 172 


the necessary power of refraction of the enclosed meniscus until it 
becomes equal to the refractive index of air, or unity. Then it be- 
comes possible to eliminate the central meniscus and replace it with 
an air lens, simplifying the construction and making the lens easier 
to manufacture. 

Lenses made according to this design were introduced by Goerz 
about 1900 in two series, the Celor having a relative aperture of F/4.5 
and the Syntor with a relative aperture of F/6.8. The spherical and 
astigmatic errors of this design are small and can be practically 
eliminated or at least to the same degree as in the three glass element, 
but complete removal of coma is impossible. While coma may be 
entirely absent in a cemented element of three glasses, complete 
comatic correction is impossible for an element of two separate lenses 
without the loss of anastigmatic flatness of field, hence objectives of 
this class have a certain amount of coma which increases as the aper- 
ture is enlarged. Lately calculation has shown that better comatic cor- 
rection may be secured by departing slightly from absolute symmetry 
in design and an objective of this type but not completely symmetrical 
has been introduced by Goerz as the Dogmar (Fig. 80). The single 


Sii- —}- 


Fic. 80. The Dogmar of Goerz Fic. 81. The Gaussian Objective 


elements of the two-lens system can be used separately only with very 
small stops, as its corrections are not nearly so complete as the ce- 
mented three glass element. ? 

The Gauss Lens.—The Gauss objective is derived from the Gauss 
telescope and in its simplest form consists of two menisci separated 
from each other by an air space and having their concave sides fac- 
ing the incident light (Fig. 81). This type of construction is par- 
ticularly favorable for reducing spherical aberration and may also be 
well corrected chromatically and astigmatically. 

In 1896 Paul Rudolph calculated for Carl Zeiss of Jena an ob- 
jective along the lines of the Gauss construction, which was placed 
upon the market as the Planar (Fig. 82). The Planar differs from 


120 PHOTOGRAPHY 


the essential form of the Gauss construction as shown in Fig, 81 by 
the replacement of the inner menisci by two cemented lenses. These 
two lenses are made of glasses having identical refractive index but 
different dispersions. ‘Therefore the inner cemented lenses act as a 
single lens so far as the refractions of any single color are concerned, 
while owing to the difference in the dispersive values of the two 
glasses the amount of chromatic aberration may be altered simply 
by changing the curves of the two cemented surfaces which separate 
the two mediums of different dispersion. In this way the chromatic 


Fic. 82. Rudolph’s Planar 


aberration of the two cemented lenses may be made to equalize the 
chromatic aberration of the two outer lenses so that satisfactory color 
correction may be obtained at the same time that the astigmatic and 
spherical errors are corrected. The relative aperture of the Planar 
is F/3.5, but owing to the presence of considerable coma the defini- 
tion at this aperture is not critical and stopping down is necessary for 

critically sharp definition. The Planar is no longer made, having 
been replaced by the unsymmetrical anastigmats which Lave approxi- 
mately equal speed and superior correction. 

H. Kollmorgen was the first to show that the Gaussian objective 
might be chromatically corrected without altering its form or affecting 
its astigmatic correction. Kollmorgen’s method was to make each 
combination of an anomalous glass pair; i.e. the coefficient of refrac- 
tion of the collecting lens with low dispersion must be as large or 
larger than the dispersing lens. A construction from calculations by 
Kollmorgen was placed upon the market by Hugo Meyer of Goerlitz 
in Germany as the Aristostigmat (D. R. P. 125,560). This objective 
(Fig. 83) is made in several series from F/4.5 to F/6.8. The strict 
symmetrical construction is departed from in the larger aperture 
series in order to obtain better comatic correction. Especially notable 
is the large flat field of this objective which is a characteristic of the 


Peer PHOTOGRAPHIC OBJECTIVE 121 


Gaussian construction. There is a considerable amount of coma, : 
however, which limits the effective aperture when critical definition is 
required. 


Fic. 83. Kollmorgen’s Aristostigmat 


Identical in construction with the Meyer Aristostigmat is the Ross 
Homocentric which is made in four series: F/5.6, F/6.3, F/6.8 and 
F/8. The single elements of the last three series may be used alone. 

The Plaubel Double Orthar appears to be of similar construction. 
It is made in two series with relative apertures of F/6.3 and F/6.8. 

The Omnar of Emil Bush is along the same lines but the corrections 
were worked out in a different manner. In the Omnar the two glasses 
have different indices of refraction, the negative lens having the 
higher refracting power than the positive, while in the previously de- 
scribed construction both lenses have practically identical indices of 
refraction but varying dispersions. The Omnar can be made without 
the use of any of the newer varieties of Jena glass and is completely 
anastigmatic. The chromatic and spherical aberrations are well cor- 
rected and the objective is notable for its large flat field. Coma, how- 
ever, is present as in all lenses of this construction. 

In 1920 Taylor, Taylor and Hobson and H. W. Lee patented an . 
improved form of the Gaussian construction which was later intro- 
duced as the Cooke Series O, Opic F/2 (B. P. 157,040 of 1920). 
This objective (Fig. 84) consists of six components symmetrically 
arranged but not identical, two of which are simple meniscus col- 
lecting lenses of dense barium crown 1p 1.6. Between these lenses 
are two compound dispersive components each consisting of a plano- 
convex collecting lens of light flint cemented to a plano-concave dis- 


pope AS ere 


122 PHOTOGRAPHY 


persing lens of barium crown. The ditterence in the refractive in- 
dices of the two cemented lenses must be at least 0.03. The lens has 
an aperture of F/2 with an angle of view approximately equal to 
lenses of similar focal lengths working at F/4.5. Since its introduc- 


Fic. 84. Lee’s T. T. H. Opic 


tion it has been extensively employed in press photography, for 
theater snapshots, and other work requiring short exposures in poor 
light. 

The Plasmat of Dr. Paul Rudolph.—In 1920 Dr. Paul Rudolph, the 
designer of the Planar, Unar, Tessar, Protar and numerous other 
objectives, calculated a symmetrical objective known as the Plasmat 
which was placed upon the market by Hugo Meyer of Goerlitz and 
Suter of Basel, Switzerland. The Plasmat (Fig. 85) consists of two 


| 
| 


Fic. 85. Rudolph’s Plasmat 


similar combinations, each of which is composed of a plano-convex 
or convexo-concave collecting lens cemented to a double-concave dis- 
persing lens and_a thin meniscus separated from the cemented pair 
by an air space. The greatest curvatures are all concave to the dia- 
phragm. The relative aperture is F/4 for the double lens composed 
of two F/8 elements of equal focal power and F/5.5 for a double 
objective composed of two elements of unequal focal lengths. 


~ a 


THE PHOTOGRAPHIC OBJECTIVE 123 


It is claimed that the Plasmat is more completely corrected for 
spherical aberration than any other anastigmat. In the ordinary 
anastigmat the spherical correction is not equally perfect for each 
color, the correction being less perfect for blue than for yellow. Con- 
sequently the ordinary anastigmat does not possess the depth of focus 
that a Plasmat of the same focal length and aperture has because the 
depth of focus depends on the spherical correction which is more 
completely corrected in the Plasmat than in other anastigmats. 

So radical a claim has not been allowed to pass without criticism. 
Among the notable criticisms we may mention that of Zschokke 
(Photo. Ind., 1921, p. 257), who states that the greater apparent 
depth of focus is due solely to the presence of a small amount of un- 
corrected chromatic aberration. 

Steinheil’s Unofocal.—The usual method of correction for spheri- 
cal, astigmatic and other aberrations is, as we have seen, the opposi- 
tion of powerful lenses of different powers so as to secure compensa- 
tion of the opposing kinds of aberration. In the Unofocal calculated 
by Steinheil and covered by D. R. P. 133,957 the conditions necessary 
for securing an anastigmatically flat field are met with lenses of very 
low power free from excessive curvatures and glasses of great thick- 


Fic. 86. Steinheil Unofocal 


ness, both of which react favorably on the performance of the ob- 
jective. , 

In the Unofocal (Fig. 86) there are two double-concave dispersing 
lenses interposed between two exterior collecting lenses, the general 
appearance of the objective closely resembling the Celor and Syntor 
of Goerz. The construction of the Unofocal, however, is based upon 


124 PHOTOGRAPHY 


an entirely different principle and should not be considered to be in 
any way allied to the objectives just named. In the first place all 
four lenses are made of glass having practically the same refractive 
index and are all of the same focal power. If the lenses were placed 
in contact this would result in complete neutralization, so that the 
system would no longer have a positive focus, but by slightly separat- 
ing the elements a converging system having a positive focus is se- 
cured. Spherical aberration is corrected by two refractions in the 
same direction assisted by the relations of the lenses and the facing 
surfaces. Flatness of field is made possible by the use of glass having 
nearly the same refractive index, while achromatism is secured by 
suitably balancing the dispersions of the glasses. 

This design makes possible the construction of an anastigmat which 
owing to its shallow curves and simple construction is easily manu- 
factured. The performance of the lens is excellent. 

The Unofocal is made by Steinheil in several series, the maximum 
having.a relative aperture of F/4.5. 

There are several other lenses on the market which are based upon 
the Steinheil principle. Among those which depart but little from the 
original design the Gundlach anastigmat may be mentioned. This has 
a relative aperture of F/6.3. 

The Graf Anastigmat and Variable.—The Steinheil Unofocal is a 
symmetrical lens, the two glass pairs on either side of the diaphragm | 
being identical. In 1911 an American, Christopher Graf, designed 


Fic. 87. Graf Anastigmat 


an unsymmetrical anastigmat based upon the Steinheil design, which 
was introduced several years later as the Graf Anastigmat. 

In this objective (Fig. 87) the two inner dispersive lenses are 
identical with respect to each other and are symmetrically placed. 
The exterior collecting lenses, however, while similar are not identi- 


foe PHOTOGRAPHIC OBJECTIVE 125 


cal either in form or position. The refractive index for the G line is 
the same for all four lenses (ng I ==1.6253). The refractive index 
for the D line is slightly higher for the collecting lenses (np = 1.6110) 
than for the dispersive lenses (1p = 1.6095). 

‘An especially notable feature of this construction is the most ex- 
cellent quality of diffusion secured by lessening the separation be- 
tween the dispersing lenses and the front collecting lens. ‘The ob- 
jective is mounted in an adjustable mount in order that the amount of 
diffusion may be regulated by the worker to suit the demands of the 
subject. The displacement of the coilecting lens lengthens the focus 
of the system, and since the actual aperture remains unaltered, the 
speed consequently becomes less. The relative aperture of the ob- 
jective when adjusted for full correction is F/3.8; when set for the 
maximum usable degree of diffusion, F/4.5. 

The Beck Isostigmar and Neostigmar.—These lenses. were intro- 
duced by Beck Limited of London, the former in 1906 and the latter 
in 1910. ‘They differ from the lenses which we have previously de- 
scribed in that they do not obey the so-called “‘ Petzval condition.” 
According to the Petzval condition in order to secure an anastigmatic 
objective with a flat field the sum of the focal powers of the individual 
surfaces, when divided by the product of the refractive indices on 


Fic. 88. Beck’s Ysostigmar 


either side of the surface, should be zero. These lenses were worked 
out on certain lines which do not take the Petzvel condition into con- 
‘sideration, so that these lenses do not obey the same. 

The Isostigmar is a five-lens system (Fig. 88), all of the lenses being 
of low power and uncemented. The two exterior are collective; the 


three interior dispersive. By careful calculation of the curves and the 
10 


126 PHOTOGRAPHY 


separation of the components an objective can be calculated which is 
very well corrected for spherical and astigmatic errors together with 
coma. It is made in several series with a maximum aperture of F/3.5 
in the shorter foci and F'/4.5 in the longer. 

The Neostigmar is a later introduction and is simpler in construc- 
tion. The single combinations are better and can be used at a larger 
aperture. It is a four-lens objective of unsymmetrical construction 
(Fig. 89). The two exterior lenses are collecting; the interior dis- 
persing. Convertibility is secured by removing the third or fourth 
lens ‘and using the remainder of the system. The Neostigmar is made 
in several series with apertures of F/6 and F/7.7. One of these 
series is especially notable for the large field covered with satisfactory 
definition at F/6.3 and may be used as a wide-angle lens. 

The Dallmeyer Stigmatic.—The opposition of a spherically cor- 
rected glass pair and an astigmatically corrected glass pair, a method 
adopted by Rudolph in his early Protars, suffers from the disadvantage 
that simultaneous correction for astigmatism and spherical aberration 
is impossible at a large aperture. The conditions necessary to the re- 
moval of spherical aberration ipso facto increase the amount of astig- 


Fic. 89. Beck’s Neostigmar Fic. 90. Dallmeyer Stigmatic 


matism; hence the objective cannot be well corrected for large aper- 
tures. 

In the Stigmatic, calculated for J. H. Dallmeyer by H. L. Aldis and 
covered by British Patent 16,640 of 1895, this difficulty is overcome in 
a novel way. The complete objective (Fig. 90) consists of two new 
glass pairs to one of which has been added a strongly converging me- 
niscus lens separated by an air space. The two cemented surfaces of 
the two elements formed of a new glass pair enable astigmatic correc- 
tion to be obtained, while the addition of the thin, strongly converging” 
meniscus lens enables the opposite spherical effect to be compensated 
for without affecting the astigmatic correction. The Stigmatic could 
therefore be made to work at larger apertures than the Rudolph ob- 


THE PHOTOGRAPHIC OBJECTIVE 127 


jective. It was formerly made in several series, the largest with an 
aperture of //4, but is now made in only one series with an aperture 
of F/6. : 

Rudolph’s Early Protar Lens.—Earlier in this chapter we referred 
to the different methods adopted by Emil Von Hoegh and Paul 
Rudolph for the correction of astigmatism in photographic objectives. 
In the pages immediately preceding we traced the development of the 
symmetrical objective from the double objective formed by the union 
of two of the three glass elements as patented by Von Hoegh and 
Goerz in 1893 (D. R. P. 74,437). In the following pages we propose 
to trace the development of the unsymmetrically constructed objective 
from Rudolph’s early Protar lens. 

Paul Rudolph of the Carl Zeiss Werkstatte at Jena was one of the 
first to take advantage of the newer glasses introduced by the Jena 
Glass Works in the construction of photographic lenses. The first of 
Rudolph’s anastigmatic objectives with which we are concerned was 
introduced in 1890 under the name Protar and covered by D. R. P. 
56,109 (U. S. P. 444,714, 1891). This objective (Fig. 91) con- 


Fic. 91. Rudolph’s Unsymmetrical Protar 


sisted of two glass pairs, one being the old normal glass pair made 
from the old glasses, the other the abnormal glass pair made from the 
newer varieties of Jena glass. One of the combinations has a positive 
astigmatic difference, i.e. the focal length of the rays in the primary 
section is greater than in the secondary section; while the other com- 
bination has a negative astigmatic difference, the focal length of the 
trays in.the primary section being Jess than in the secondary section. 
By proper construction, the two opposite effects may be so balanced as 
to compensate one another so that there is no sensible astigmatic differ- 
ence when the complete objective is said to be an anastigmat. 

In the normal glass pair the refractive index of the positive lens is 
lower than the adjacent negative lens, while in the abnormal glass pair 
the positive lens is of higher refractive index than the negative. Both 


128 PHOTOGRAPHY 


components are separately achromatized, but are not necessarily of the 
same focal power, as one component may be strongly positive while 
the other may act simply as a corrective system for the whole. This 
construction because of its relatively small aperture is no longer made, 
except for wide-angle objectives where its extensive field makes it very 
suitable. The wide-angle Protar has this form. 

The Introduction of Air Spaces—the Unar.—Some nine years 
after, Rudolph realized the advantages to be gained from replacing the 
ccmented surfaces by air spaces having opposite power. ‘This princi- 
ple was applied to the construction of the Unar introduced by Carl 
Zeiss in 1899 from D. R. P. 134,408 (U. S. P. 660,202 of 1900). 
Several forms of objectives constructed on this principle were de- 
scribed by Rudolph, from which that illustrated in Fig. 92 was adopted 
for the commercial product. 

It will be observed that this objective consists of two pairs of glass 
surfaces, each pair consisting of two surfaces which face one another, 


Fic. 92. Rudolph’s Unar 


i.e. which belong to two consecutive lenses and are separated by an air 
space but not by the diaphragm. The powers of both pairs of facing 
surfaces are of opposite sign, the first having a positive astigmatic 
- difference, the second a negative astigmatic difference. The effect of 
the two pairs of facing surfaces of opposite power is similar to the 
result obtained in the early Protar of 1890, by the difference in 
the refractive indices of the crown and flint lenses in the cemented 
components of the doublet. Just as the two cemented surfaces, one 
convex to the medium of higher refraction, the other to that of lower 
refraction, produce opposite effects of astigmatism, so do the pairs of 
facing surfaces of opposite sign produce the opposite effects of astig- 
matism which may be completely compensated by making the two op- 
posite effects of equal power. The introduction of the air space en- 
ables a greater degree of correction to be obtained, as the number of 
elements of correction are considerably increased. : 


fo PHOTOGRAPHIC OBJECTIVE 129 


The aperture of the Unar was F/6. It is no longer made, having 
been replaced by the Tessar introduced by Rudolph in 1902. 

Combination of Air Space and Cemented Surface—the Tessar.— 
In 1902 Rudolph patented (D. R. P. 142,294, U. S. P. 721,240, 1903) 
the objective issued by Carl Zeiss as the Tessar. The Tessar may be 
described as a combination of the early Protar and the Unar. It 
consists (Fig. 93) of four lenses divided into two groups which are 


| 
| 


Fic. 93. Rudolph’s Tessar 


separated by the diaphragm. The first group contains a collecting 
dnd a dispersing lens separated by an air space having a negative 
effect. The second group consists of a cemented negative and a posi- 
tive lens, the cemented surface having a positive effect. 

In the early unsymmetrical Protar lens the opposite effects by which 
astigmatic correction is secured are derived solely from the action of 
the cemented surfaces. In the Unar the said correction is based on 
the opposite powers of the two pairs of facing surfaces. In the 
Tessar these opposite astigmatic effects are obtained by giving to the 
power of the cemented surface the opposite sign to that presented by 
the facing surfaces of the other group of lenses. . 

A negative cemented surface may be used with a positive pair of 
facing surfaces, but this combination is not so effective as the use of a 
positive cemented surface and a pair of facing surfaces having a nega- 
tive effect.? 

The first and fourth lenses are constructed of glass with a high re- 
fractive index (1.61132) and are positive; the second and third are 
dispersive in effect, the second having the higher refractive index. 

1A cemented surface in opposition to a pair of facing surfaces was not 


wholly original with Rudolph, since it was employed by Petzval in the Portrait 
Objective. In this objective, however, the two powers have the same sign. 


130 PHOTOGRAPH 


On account of its excellent corrections which place it among the 
best of photographic objectives, and its relatively simple construc- 
tion which makes it more easily manufactured than other lenses of 
more complex construction, the Tessar construction has been widely 
copied and since the patents have expired in practically all countries 
there are numerous lenses by various firms which are essentially identi- 
cal with the Tessar. The following lenses are of the Tessar type: 


ALGGs ern v. cea ba aunt ee Series 1... cl.540)0 eee F/4.5 
PLPHeMman | ass wheres tae Ernon ..... coweseen eee F/4.5 
Oda EAS lento vive te 2 eee Anastigmat .o... 7) ee F/4.5 
Koristka ni. as kale Ge ee Sidefan ... Jcsess2 eee F/4.5 
Rirawag § isin, Sete ee es Trianar ....s50sc0. ee F/4.5 
Li arenas UAT MTT ug SMa I Dialytar 23. \iee.ee eee F/4.5 
Plawbel <5 octavian ane: ve eae Anticoma:. . cs .6sa eae F/4.5 
RudersGorti: <5 tos... a see ee Acoma’ ....¢: 00 een ee F/4.5 
SalmoOwach (ss. cece nie eee tee Phoebus ©. 230.9 eee F/4.5 
Schneidet ees See ee ee Nenar ae ‘gestae eee F/4.5 
FT wanty YO see ee Transpar’ :.\vos a5 nee F/4.5 
Wollensale: i cco ye eee Velostigmat Ser. Ilias F/4.5 


Combination of Air Space and Cemented Surfaces—Further De- 
velopments.—In 1912 Dallmeyer Limited patented (B. P. 27,518 of 


Fic. 94. Dallmeyer Serrac 


1912) an objective on the principle of the Tessar which was introduced 
commercially as the Serrac. This objective (Fig. 94) is identical in 
design with the Tessar but the glasses are changed and necessarily the 
radii. In the Tessar the first and fourth glasses are the same, having 
high dispersion, while the second is of glass having a lower refractive 


nth. , SiS eee 
ay a. ow 


ee te 


Pri PHOTOGRAPHIC: OBJECTIVE 131 


index but higher than the third lens, which is made of glass with a 
medium refractive index. In the Serrac, as in the Tessar, the first 
and fourth lenses ate of glass with a high refractive index but, unlike 
the Tessar, the second and third lenses are composed of identical 
glasses having a medium refractive index. The use of the same glasses 
for both of the dispersing lenses makes it possible to secure complete 
astigmatic correction with shallower radii than is possible in similar 
systems in which one of the lenses is composed of glass with a high 
refractive index and low dispersion and the other of low refraction 
and high dispersion and this reacts favorably on the other corrections. 

The Dalmac F/3.5 issued by Dallmeyer Limited is a recalculation 
of the Serrac, only such changes having been made as were necessary 
to secure satisfactory performance at the larger aperture. 

In 1913 Ross Limited patented the X-press (B. P. 29,637 of 1913), 
a lens based upon the same principle as the Zeiss Tessar but differing 
from it in the use of a rear element consisting of three glasses instead 
of two (Fig. 95). The first lens of the rear component is made of 


Fic. 95. Ross X-press Fic. 96. Gundlach Radiar 


glass with low refraction, the second with medium refraction and the 
third with high refraction. The gradual increase in the indices of 
refraction together with the two cemented surfaces available enables 
the complete objective to be so corrected that the usual errors are at a 
minimum for this type of construction. Both cemented surfaces in 
the rear component are collective in effect. 

In 1921 the Gundlach Manhattan Optical Company of Rochester, 
N. Y., introduced the Radiar (Fig. 96), a lens resembling the Ross 
X-press in the use of a rear element of three glasses but differing from 
it in that one of the cemented surfaces is collective while the other is 
dispersive. The first and fourth lenses are of light barium crown 


132 PHOTOGRAPHY 


(mp 1.570908, v 57.2); the second and fifth are of light flint (up 
1.58132, v 41.0) while the third lens is a boro-silicate crown (np 
1.5109, v 57.2). The three glass element was adopted because it 
allows a different selection of glasses and affords more latitude in 
balancing the powers of the component parts of the system. 

The Uncemented Triplet—the Cooke Lens.—The triplet design 
was applied to the construction of photographic objectives at a very 
early date. As early as 1841, Andrew Ross made for Fox-Talbot a 
triplet which consisted of a concave dispersing lens of flint, interposed 
between two equi-convex collecting lenses of crown, and some years 
later Sutton worked out a similar construction, while in 1860 Dall- 
meyer issued his “ Triple achromatic lens” in which each of the three 


components consists of a cemented collecting and dispersing lens sepa- — 


rately corrected for chromatic aberration. Many other examples of 
the triplet construction will be found in the work of Von Rohr, which 
has already been mentioned several times as being the best source of 
information on the development of the objective. 

The Cooke lens patented (B. P. 22,607/93-15,107/95—1,699/1899, 
U. S. P. 540,122 of 1895 and 586,052 of 1896) by Mr. H. Dennis 
Taylor, however, cannot properly be considered as a development of 
the earlier triplets. Although both are triplets, the construction of the 
Cooke is based on a radically different principle, and the two can be 
said to be similar only in outward appearance. In none of the earlier 
triplets is the power of the negative lens more than a small fraction of 
the sum of the collecting lenses, and the idea of utilizing the dispersing 
lens for flattening the image, and correcting marginal astigmatism, ap- 
parently never occurred to these men. 


Fic. 97. Cooke Triplet 


The Cooke lens is a simple triplet (Fig. 97) consisting of a double- 
concave dispersing lens interposed between two double-convex collect- 
ing lenses. The two collecting lenses are identical and are designed 
to be practically free from coma; this result being secured by the use 
of an intermediate form of lens in which the inward coma of one 


fie PHOTOGRAPHIC OBJECTIVE 133 


surface is neutralized by the outward coma of the opposite surface. 
The burden of correcting the entire system is thrown upon the central 
dispersing lens, which fulfills a threefold office: (1) It flattens the 
final image and corrects marginal astigmatism, producing a flat astig- 
matic field; (2) it corrects the color aberrations of the convergent 
lenses and makes the complete system achromatic; (3) it corrects the 
residual spherical aberration of the two collecting lenses and renders 
the final image aplanatic. The action of the negative lens in securing 
a flat field, free from astigmatism, may be explained with the aid of 
Fig. 98. JL and N are respectively a positive and negative lens of 


Wi 


as ae 


i ; 
\ 


Fic. 98. Action of the Central Diverging Lens 


equal focus and made of the same materials. With primary sections 
of the oblique pencils, the image formed by the positive lens, L, is 
curved spherically, p—-P, but the interposition of the negative lens, N, 
throws back the image to the plane Q—Q’ which is flat because the 
curvature errors of the positive lens, L, are exactly neutralized by the 
opposite curvature of the negative lens, NV. | 

Owing to the fact that this arrangement would result in consider- 
able distortion, and to the fact that its corrections to a certain extent 
depend upon the distance of the subject, the single collecting lens is 
divided into two which are alike in power and shape but turned in 
opposite directions. The positive focal power of the two converging 
lenses is approximately equal to the negative focal power of the 
central dispersing lens. 

Considering the simplicity of construction and the limited number 
of elements which the designer possessed to correct the usual aberra- 
tions, the Cooke lens is well corrected and at the time of its introduc- 
tion (1895) surpassed the anastigmats then known in sharpness of 
definition over the usable angle of view. Especially notable is the al- 
most complete absence of coma. 

The Cooke triplet construction is followed by Taylor, Taylor and 


hw a 
4 


134 PHOTOGRAPHY 


Hobson of Leicester, England, in the construction of several series of 
lenses ranging in speed from F/3.1 to F/8. The more rapid series 
intended for portrait work in the studio naturally are corrected for a 
much smaller field than those of smaller relative aperture. Owing to 
the simplicity of the Cooke triplet construction, its performance and 
to the fact that the patents have now expired, numerous manufacturers 
issue under various trade names objectives based upon the same princi- 
ple. We mention a few: 


Aldis= tS Sil ee eee Series O.. Ji2. bee F/3 
GOGrs OOS oe eee eee Hypar ..¢4.. 28 F/3.5 
Rietschel—Portrait Anastigmat, . 
Rodenstock “Wiest, se eee Eurynar (Older models)....F/4.5 
Rudersdort > 3.4... eeee ee ee Tular. .... 455 eee F/6.3 
Salmotrach 2.0305 sca 2 Bee Orioti (ica) eee F/4.5 
Staebel) i) S25 es aoe ee Kaloplast, 

Steinheil 3) et a8 coke eee Cassar). ie ee F/3.5 
LCisS .... . Poe Oa e Triotar 4. .g<, seo oa eee F/3.5 


Development of the Triple Objective after H. D. Taylor—the 
Cooke Aviar.—In 1918 Arthur Warmisham patented (B. P. 113,590 — 
of 1918) a modified form of the Cooke lens which consists of four 
simple spaced lenses, two of which are collective and two dispersing. 
It is essentially a Cooke construction, the idea of a split divergent lens 
having occurred to Mr. H. D. Taylor in 1898, who was granted a 


Fic. 99. Cooke Aviar—Shorter Foci—Longer Foci 


patent for a modification of the triplet in which the central dispersing 
lens was developed into two similar lenses of lower individual power 
(B. P. 12,859 of 1898). This objective, however, had no advantages 
over the earlier triplet and was abandoned. By making a special 
study of coma,-Warmisham was able to develop an objective of this 
type which has a larger flat field than the triplet. 

In the shorter foci (Fig. 994) the two collecting lenses are made 
of highly refractive dense baryta crown (up 1.6116, v 56.4) and are of 


feet OL OGRAPHIC OBJECTIVE 135 


identical power. ‘The two dispersing lenses are of light flint, but have 
slightly different values (Lens 2, np 1.5682, v 43.4; Lens 3, up 1.5502, 
UV 45.8). 

For objectives of longer focus the collecting lenses are made of 
dense baryta crown (7p 1.6116, v 56.4) as in the case of the shorter 
foci objectives, but the dispersing lenses are made of heavy flint (np 
1.6206, v 36.2). Thus in the case of the short-focus objectives the 
refractive index of the collecting lenses is the higher, in the case of 
the long-focus objectives the reverse is the case, the refractive index 
of the dispersing lenses being higher than that of the collecting lenses 
while the dispersion is less. In both cases the shallower faces of all 
four lenses are turned towards the diaphragm. This necessitates a 
shallower curve for the inner surface of the back positive lens, which 
reacts favorably on the curvature of field, marginal astigmatism and 
comatic correction over the entire field. 

Development of the Triplet Objective after H. D. Taylor—the 
Aldis Lens.—Three objectives of this firm demand attention. Series 
Ila F/6.3 (Fig. 100) may be regarded as a development of the triplet 


Fic. 100. Aldis Series Ila 


construction of H. Dennis Taylor as described in British Patent No. 
1699 of 1899. The central dispersing lens is separated from the front 
collecting lens by a small air space having a positive effect. The rear 
collective lens is separated from the front element by the diaphragm. 
In the original Cooke triplet the collecting lenses are approximately 
equal in focal power, here the front collecting lens is much more 
powerful than the rear. 

Series II and Series III may also be considered as having been 
evolved from the Cooke triplet, although they fall into entirely differ- 


ent classes. In both of these (Fig. 101) the front collecting lens is 


cemented to the central dispersing lens while the rear collecting lens of 
low power is placed some distance from the cemented element. In 
both lenses the collecting lenses are of baryta light flint; the dispersing 
lenses of baryta flint. Series II has a relative aperture of F/6; Series 
mot F/7.7. 


136 PHOTOGRAPHY 


Triplets with a Pair of Collecting Cemented Surfaces—the Heliar. 
—In 1900 Hans Harting calculated for Voigtlander an objective 
which is essentially a development of the simple triplet of H. Dennis 


Fic. tor. Aldis Series II and III 


Taylor, which was introduced commercially as the Heliar under D. R. 
P. 124,943 of 1900 (U. S. P. 716,035 of 1902, B. P. 22,962 of 1900). 
In the Heliar (Fig. 102) the triplet construction is plainly evident 


| 


Fic. 102. Harting’s Heliar 


from the general similarity of the design. The development made 
by Harting consists in replacing the single collecting lens of the 
original triplet of H. Dennis Taylor by a cemented element of two 
glasses. The objective thus becomes a five glass system, the two ce- 
mented lenses being an anomalous glass pair, the outer dispersing 
lens being of flint (mp 1.54990, mgt 1.56547) and the inner collecting 
lens of crown (up1.61294, ngt 1.62686), while the central bi-concave 
dispersing lens is made of flint glass of lower refraction (mp 1.53644, 
ngl 1.54988). Both cemented surfaces have a collective effect. 

By increasing the number of elements of construction and the in- 
troduction of cemented surfaces the initial errors of construction are 
lessened and excessive curvatures avoided so that the corrections are 
more easily and fully carried out, resulting in better field covering 
at a large aperture. The aperture of the Heliar is F/4.5 in all sizes 
and the area of the field covered with satisfactory sharpacene is greater 
than that of the simple triplet. 


al 
= 


Peer rOTOGRAPHIC OBJECTIVE for 


The Dynar.—Two years later Harting calculated for Voigtlander 
a similar construction but with the glasses of the cemented com- 
ponents reversed in position so that all three dispersing lenses are 
placed together. This construction was introduced as the Dynar 
_ (Fig. 103) under D. R. P. 143,889 of 1902. The Dynar was made 


Fic. 103. Harting’s Dynar 


principally for hand cameras and has a maximum aperture of F/6. 

The Pentac.—While the Pentac issued by J. H. Dallmeyer from cal- 
- culations by Lionel Barton Booth is described in British Patent Speci- 
_ fication No. 151,506 as a development of the Tessar, the single col- 
lecting lens of the former being replaced by a cemented component 
consisting of a collecting and dispersing lens with collecting cemented 
surface, examination shows that the Pentac has more in common 
with the Heliar and particularly the Dynar than with the Tessar (Fig. 
104). Both the Dynar and the Pentac are five-lens systems con- 


Fic. 104. Dallmeyer’s Pentac 


_ sisting of a double-concave negative lens interposed between two col- 
 lecting elements formed of a cemented collecting and dispersing lens 


and both the exterior lens and the cemented surface are collective in 


effect. The arrangement of glasses in the Pentac is sensibly the same 
4 arrangement as the Dynar, the exterior double-convex collecting. 
lenses are of highly refractive crown (mp 1.6109) and the three nega- 
q tive lenses of flint glass with a refractive index of about np 1.5485. 


138 PHOTOGRAPHY 


By rigid calculation an objective has been calculated which has an 
unusually large aperture of //2.9 and can be well corrected up to a 
focal length of 12 inches.? 

The Ernostar.—This objective of Ernemann of Dresden is based 
upon the Cooke triplet. The front collecting (Fig. 105) lens of the 


Fic. 105. The Ernostar 


latter, however, has been replaced in the new objective by two elements, 
each composed of two cemented lenses, the front lens of each pair 
being collective and the rear lens dispersive in effect. The two ele- 
ments are separated from one another by an air space having a disper- 
sive effect. The action of this complex front component is to secure 
greater convergence of the incident rays so that the path of the ray 
after passing through the central negative component may be either 
converging or parallel and not diverging as in the original triplet of 
H. Dennis Taylor. The division of the front member into two sepa- 
rate components increases the number of elements at the disposal of 
the calculator, since there are four glasses and an air space, and this 
has enabled the corrections to be carried to a high degree of perfec- 
tion notwithstanding its large relative aperture of F/2. The spherical 
aberrations, astigmatism and zonal errors are almost completely re- 
moved and the field is flat and free from distortion. So completely 
has the chromatic correction been carried out that the objective may 
be considered to be sphero-achromatized and apochromatic.® 


Part III. THE TELEOBJECTIVE 


Principle of the Teleobjective.—If we take two lenses made of the 
same glass and having equal but opposite powers, one being negative 
and the other positive, it is evident that if they are placed in contact 
with one another the converging power of the positive lens will be 


2 Since this was written there has appeared an F/2.7 and F/1.8 by Zeiss. 


* British Patents 186,917/1921, 191,702/1922, 193,376/1922, 232,531/1924. Klug- j 


hardt, Phot. Ind., 1924, p. 1008. 


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tier HOVOGRAPHIC OBJECTIVE 139 


exactly balanced by the dispersing power of the negative lens and 
there will be no alteration in the direction of the incident ray. The 
combination thus neutralized has no real focus, or it may be said to 
have infinite focal length. However, if we separate the positive and 
negative lenses the focal length will gradually shorten until finally 
we reach the zero position where the focal distance is equal to the 
focal length of the positive lens, the negative lens then being without 
effect on the focus. The negative lens may thus be said to take a 
portion of the image produced by the positive lens and magnify it. 
The amount of the magnification depends upon the focal length of 
the objective which in the teleobjective is determined by the distance 
between the positive and the negative lenses. This distance between 
the lenses, or A, increases as the focal length decreases and vice 
versa. 

The separation of the two lenses also brings about another change. 
When the two are in contact the principal point coincides with the 
common lens vertex, but as the components are separated the prin- 
cipal point moves away from the lens in the direction of the subject. 
Since the focal length is the distance from the principal point to 
the point of intersection of the convergent rays, the distance from 
the focal plane, or the ground-glass, to the lens is less than the equiva- 
lent focal length. Hence we are able to make use of a long focus 
objective without a bellows extension of corresponding length. This 
is the principal point of value of the teleobjective. 

The Compound Telephoto Objective.—The earliest use of a negative 
lens in the above described manner is found in the Galilean telescope. 
Its first use for photographic purposes is credited by Harting to J. 
Porro in 1851.4 The matter, however, remained unnoticed by the 
optical world at large until the latter part of the nineteenth century 
when it was independently invented by several opticians and is now 
made by nearly all manufacturers of photographic lenses. 

_ As a typical example of the compound teleobjective we may men- 
tion the design patented by Dallmeyer in his English patent No. 21,933 
of 1891. The positive component of this system is the well-known 
Petzval portrait lens; the posterior negative element is a symmetrical 
double combination as illustrated in Fig. 106 and is chromatically 
and spherically corrected. The two elements are mounted in a tube 
fitted with an adjustable screw by which the separation of the posi- 


4 Optics for Photographers, English translation, p. 185. 


140 PHOTOGRAPHY 


tive and negative components may be altered to secure any desired 
degree of magnification. 

Most manufacturers are in a position to fit, to such of their ob- 
jectives as may be suitable, a negative combination similar in general 


Fic. 106. Dallmeyer’s Compound Teleobjective 


construction to the above. When ordering the negative lens the ob- 
jective to be used as the telepositive should be sent to the manu- 
facturer in order that the two may be properly adjusted. 

The advantages of variable focal length and size of image to- 
gether with short bellows extension are important features and if it 
was not for the serious disadvantage of lack of speed, which con- 
siderably limits its usefulness, the compound teleobjective would be 
widely used. It is not difficult to understand the reason for the lack 
of speed when we consider the principle upon which the teleobjective 
is based. The image formed by the positive lens is enlarged (spread 
over a larger area) by the negative lens ; therefore the intensity of the 
image is less and a longer exposure is required. The higher the 
degree of magnification the greater the exposure required. Mathe- 
matically the aperture of a teleobjective may be expressed as 


fil A 
where D is the aperture of the positive objective, 
f, the focal length of the positive lens, 


f2 the focal length of the negative lens, and 
A the separation of the positive and negative lenses. 


Furthermore some stopping down of the positive lens is nearly al- 
ways necessary in order that the aberrations of the negative lens 
(which cannot be completely corrected because the distance between 
the two elements is subject to considerable variation according to 
the requirements of the subject) do not unduly interfere with the 
central definition. This still further lengthens the time of exposure so 
that hand camera work and the photography of moving objects be- 


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~THE PHOTOGRAPHIC OBJECTIVE 141 


comes possible only in very exceptional cases. For this reason the 
compound teleobjective is of value only for a limited type of work 
and has been almost completely replaced by the modern fixed-mag- 
nification, high-speed, anastigmatic -teleobjective. 

Early Fixed-Magnification Teleobjectives.—While it is impossible 
to secure a very large aperture with the compound teleobjective, if 
we fix once for all the separation of the positive and negative ele- 
ments so that we secure a fixed degree of magnification we are en- 
abled to considerably increase the working speed of the combination 
and without any loss of definition. Strictly speaking, the first fixed- 
magnification teleobjective was the Orthoscopic lens worked out by 
Petzval and introduced commercially by Dieztler in 1856. This ob- 
jective (Fig. 107) consists of a front positive combination similar to 


Fic. 107. Petzval’s Orthoskop 


that of the regular Petzval portrait lens and a back negative com- 
bination with a bi-concave and concavo-convex. lens, the two being 
chromatically corrected so that the objective consists essentially of 
two achromats, one of which is collecting and the other dispersing. 


This rear component magnifies the image in just the same way as in 


the compound teleobjective but as the usual corrections have to 


be made for only one degree of magnification, and not an entire 


series as in the other case, it becomes possible to give to the whole 
objective an aperture considerably in excess of that which is possible 
with the compound teleobjective. 

The possibilities of the Orthoscopic construction were not realized 
at that time, however, and with the advent of the aplanat it ceased to 
be made. It was not until 1905 that the first of the modern fixed- 
focus teleobjectives was introduced, the Bis-Telar of Emil Busch 


11 


142 PHOTOGRAPHY 


This was designed by K. Martin and was composed of two cemented 
doublets (Fig. 108) and had a relative aperture of F/g and a mag- 
nification-ratio of 124. Soon afterwards Zeiss brought out the Mag- 
nar (Fig. 109). This was calculated by Rudolph and Wandersleb 
and had a magnification-ratio of 3 times with an aperture of F/1o. 
The positive component was a doublet and the rear a triplet. 


| : 
Fic. 108. Martin’s Bis-Telar Fic. 109. Zeiss Magnar 


The Anastigmatic, Fixed-Focus Teleobjective——Designers then 
began to turn their attention towards the more complete astigmatic 
correction of the teleobjective. In 1912 Ross Limited issued from 
the calculations of Stuart and Hasselkus the Telecentric, a fixed-focus 
teleobjective, the positive component of which was a cemented triplet 


Fic. 110. Ross Telecentric Fic. 111. Dallmeyer Large Adon 


and the rear component a cemented doublet (Fig. 110). This was 
issued in two series, one working at F/5.4 and the other at F/68. 
Two years later Lan-Davis patented (B. P. 1185 of 1914) an anastig- 
matic teleobjective which was introduced by J. H. Dallmeyer Limited 
as the Large Adon. ‘This objective (Fig. 111) consists of a positive 
component containing a cemented collecting and dispersing lens form- 
ing an achromatic pair but with considerable remaining spherical aber- 
ration. The rear dispersing component consists of either two or three 
cemented lenses which form an achromatic combination and are so 
corrected spherically as to compensate for the spherical aberration of 
the front member. In this way a comparatively well-corrected objec- 
tive with an aperture of F/4.5 was obtained. | 


es ae - a S - _ ‘ 


THE PHOTOGRAPHIC OBJECTIVE 143 


The same year Lionel Barton Booth calculated and patented (B. P. 
3096 of 1914) a four-lens construction.in which the members of the 
positive element were separated by an air space. This had a relative 
aperture of F/5.8 and was a notable improvement over earlier objec- 
tives of this class as regards definition and was made by Taylor, Taylor 
and Hobson in considerable. quantities for the use of the British Air 
Force during the World War. 

From the standpoint of the manufacturing optician it was desirable 
to eliminate, if possible, the air space between the two members of the 
positive element. This problem was solved by Booth, who in 1920 


Fic. 112. Booth Teleobjective 


took out two patents (B. P. 139,719 and 151,507) for fixed-focus, 
anastigmatic teleobjectives, each element of which consists of a ce- 
mented doublet (Fig. 112). This construction was introduced by J. 
H. Dallmeyer Limited in several series as the Dallon. Series VI, XVI 
and XVIII have a magnification of 2 times and relative apertures of 
F/5.6, F/7.7 and F/6.5 respectively. Series XVII has a relative 
aperture of F/6.8 and a magnification of 2% times. 


Fic. 113. Radiar Teleobjective 


A similar construction was patented by H. W. Lee (B. P. 198,958) 
and introduced by Taylor, Taylor and Hobson as the Cooke Telic. 
This has a relative aperture of F/5.5 and a magnification-ratio of 2 
times. 

The Radiar telephoto anastigmat (Fig. 113) introduced by the 
Gundlach-Manhattan Optical Company is of similar construction. 


144 PHOTOGRAPHY 


The positive member consists of a cemented doublet with a front col- 
lecting lens of barium crown and a dispersing lens of heavy flint, while 
the rear member consists of an inner dispersive lens of barium crown 
and an outer collecting lens of light flint. 

The Telegor of Goerz differs from the cbjectives described immedi- 
ately above in that the collecting and dispersing members of the rear 


ye 


Fic. 114. Goerz Telegor 


component are reversed in position, the former being on the interior 
and the latter on the exterior, separated by an air space having a me- 
niscus shape (Fig. 114). It has a relative aperture of F/6.3 and a 
magnification-ratio of 2 times. : 

In the Tele-tessar of Zeiss the rear component is composed of two 
cemented menisci and as in the case of the Goerz Telegor the positive 
member is placed nearest the diaphragm and not on the exterior as in 
the case of the Dallmeyer Dallons and the Cooke Telic (Fig. 115) (B. 


Fic. 115. Zeiss Teletessar 


P. 179,529 of 1921). The Tele-tessar has a relative aperture of F/5.5 


and a magnification-ratio of 2 times. 

In order to construct a fixed-magnification teleobjective with a mag- 
nification above two and maintain an anastigmatically flat field at the 
same time it becomes necessary to increase the number of elements. 
In the Teleros introduced by Ross Limited from calculations by Stuart 
and Hasselkus (B. P. 188,621) the rear component is a cemented 
triplet in which two negative lenses enclose a positive member of glass 


i ia 


THE PHOTOGRAPHIC OBJECTIVE 145 


having lower refraction and higher dispersion (Fig. 116). The ob- 
jective has a relative aperture of F/5.5 and a magnification-ratio of 


| 


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Fic. 116. Ross Teleros 


slightly over two times. H. W. Lee has also patented a fixed-focus 
teleobjective (B. P. 132,067) (Fig. 117) in which the rear component 
is a triplet with a positive lens of low refraction between two négative 
members of high dispersion. This is manufactured by Taylor, Taylor 
and Hobson and has a relative aperture of F/5.5 and a magnification- 
ratio of 3. As this work goes to press Messrs. Taylor, Taylor and 


- Hobson announce a new series having an aperture of F/3.5. 


== 


Fic. 117. Lee’s T. T. H. Telephoto Fic. 118. Voigtlander’s Tele-Dynar 


The Tele-Dynar of Voigtlander also has a rear element of three 
members, two of which are cemented and the other separated by an air 
space (Fig. 118). Several other manufacturers have departed from 
the simpler constructions already described, but as they are for the 
most part unknown in this country we do not propose to discuss them 
further. 

The Adon.—Before leaving the subject of the teleobjective mention 
should be made of a construction invented by Dallmeyer and utilized 
in the construction of the Adon. 

If the positive and negative elements of a tele-compound are sepa- 
rated by a difference equal to the difference of their focal lengths, 


146 PHOTOGRAPHY 


incident parallel rays emerge parallel and an ordinary objective fo- 
cussed for parallel rays when applied to the rear of this combination 
will form an image at the focal plane of the ordinary objective, the 
magnification of the image depending upon the ratio of the focal 
lengths of the positive and negative lenses (Fig. 119). To maintain 
the actual F value of the photographic objective for any degree of 
magnification the parallel pencil emerging from the magnifying sys- 

tem must be as large as the aperture of the objective to which it is ap- 
plied, therefore the exterior positive element of the enlarging system 
must be as many times greater in diameter as the lineal degree of 
magnification desired. Thus we work without loss of speed, the 
effective aperture being the same as that of the objective alone. This 


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Equivalent. focal length 


aoe 
Negative lens forms virtual image of real image 
formea by positive lens at f, 


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Fig. 119. Dallmeyer’s Adon 


principle, however, can only be used with objectives of moderate 
diameter and for low degrees of magnification. 


GENERAL REFERENCE WorRKS 


Eper—Die Photographischen Objectiv. 
FasrE—Encyclopedique de Photographie. 
GLeEICHEN—Lehrbuch der Geometrischen Optik. 


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Pot 

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THE PHOTOGRAPHIC OBJECTIVE 147 


GLEICHEN—Theorie der Modernen Optischen Instrumente. (The English 
translation by McElwain and Swan contains a table of modern objec- 
tives which is not found in the original German edition.) 

Hartinc—Optics for Photographers. (English translation by Fraprie.) 

- LumMMerR—Contributions to Photographic Optics. (The English translation by 
Thompson contains two chapters on British objectives which are not 
found in the original.) 

Puyo anp Putticny—Les Objectifs Anachromatiques. 

TURRIERE—L’Optique Industrielle. 1920. (The most complete work on the 
later anastigmats.) 

Von Rour—Theorie und Geschichte der Photographischen Objektiv. 


CHAPTER Wi 
THE PHOTOGRAPHIC EMULSION 


Introduction.—Properly speaking, the use of emulsions in photog- 
raphy dates from the publication of the first practical method of pre- 
paring collodio-bromide emulsion by Sayce and Bolton in September 
1864 but it is in connection with gelatine that the term emulsion is 
generally associated. The gelatine emulsion which has played such 
an important part in the development of photography dates from the 
investigations of an English amateur, Dr. Richard Leach Maddox, 
whose paper describing the preparation of a sensitive gelatine emul- 
sion was published in the British Journal of Photography for Sep- 
tember 8, 1871. His method, however, was not a practical one and 
gelatine emulsion on a basis similar to that now in use did not appear 
until several years later. Although a gelatine emulsion had been © 
placed upon the market as early as 1873 by Richard Kennett, gelatino- 
bromide emulsion of practical utility may be said to have first ap- 
peared in 1878 after the discovery of the great increase in sensitive- 
ness to be secured by the application of heat to the finished emulsion. 
In the meantime three very important points had been cleared up. 
King and Johnson had shown the necessity for the removal of the 
soluble salts from the emulsion and indicated means of effecting this ; 
the last named worker had also shown the importance of using an 
excess of soluble bromide rather than an excess of silver salt; while 
Bolton had suggested that the emulsion be formed in a small amount 
of gelatine and the remainder added at a later stage—a method which 
became very valuable after the uence of digestion processes with 
heat. 

As is fairly well known, the gelatine emulsion which forms the 
sensitive coating of our plates and films consists primarily of a highly 
sensitive form of silver bromide and gelatine. If silver bromide is 
formed in aqueous solution by the double decomposition of a soluble 
bromide, as potassium bromide, and silver nitrate and the solution 
allowed to stand a short while, the silver halide will begin to pre- 
cipitate upon the sides and bottom of the vessel. However, if the 


148 


Pe eee RE Ee OO SN ee TO ee ee Ee ey ee ee eS a ee eee eS Be 


THE PHOTOGRAPHIC EMULSION 149 


silver bromide is formed in the presence of an aqueous solution of 
gelatine instead of water the solution is at first clear and slightly 
opalescent and on standing becomes milky or creamy. On standing 
the silver halide does not precipitate out of solution, as in the case of 
water, but remains in a homogeneous state. This mixture of finely 
divided silver halide and gelatine is termed gelatino-bromide emulsion. 
It is not really an emulsion, however, in the sense in which that term 
is used in colloid chemistry, but a solution of gelatine carrying in sus- 
pension minute crystals of solid silver halide. In its simplest form an 
emulsion may consist purely of silver bromide and gelatine, but at 
times a small percentage of another halide, chiefly the iodide, but 
sometimes the chloride, may be added. The available evidence at the 
present time indicates that in such cases the crystals of silver iodide, or 
chloride as the case may be, are held in suspension within the silver 
bromide and neither combine chemically with the latter nor exist 
separately as individual crystals. The processes of emulsion making 
are therefore concerned with the formation of a uniform, homo- 
geneous suspension of a sensitive form of silver bromide in a solution 
of gelatine. 7 

The Two Classes of Emulsion.—Sensitive emulsions may be divided 
into two classes: (a) those in which the silver halide is formed in the 
presence of an excess of silver nitrate and (b) those in which the 
silver halide is formed in the presence of an excess of the soluble 
halide. Aside from wet collodion, the first class consists principally 
of emulsions for positive printing out processes such as collodio- 
chloride and gelatine P. O. P. or similar silver printing papers which 
produce a visible image upon exposure. The function of the excess 
silver salt is to act as an absorber for halogen. The second class 
includes both negative and positive emulsions for development and 
may be further divided into two classes: (a) those which are used 
without further treatment after emulsification and (b) those which 
are submitted to a process of digestion, known technically as ripening, 
for increasing the sensitiveness and the density-giving powers. This 
process of ripening consists either of treating the emulsion at rela- 
tively high temperatures or in the use of ammonia, and will be dis- 
cussed in greater detail elsewhere. It is sufficient to say for the 
present that in the preparation of emulsions for positive printing, 
where a high degree of sensitiveness is unnecessary, ripening plays 
little or no part, the silver bromide, or silver chloride, being emulsi- 


150 PHOTOGRAPHY 


fied in such a way as to obtain a very fine grain. In the preparation 
of highly sensitive emulsions for negative processes, however, ripen- 
ing plays a very important part. 

General Outline of Operations in Emulsion Preparation.—The gen- 
eral outline of the processes involved in the preparaten of gelatine 
emulsion is as follows: 

1. The gelatine is allowed to swell in cold water ee finally dis- 
solved by the application of heat. 

2. The soluble halide, or halides, are dissolved in water, 

3. The required amount of silver nitrate is dissolved in water. 

4. The solution of silver nitrate is added to the colloid medium. 

5. The solution of soluble halide is next added to the colloid 
medium. 

6. The silver salt and soluble halide unite by double decomposition 
to form a silver halide. Thus in the case of silver nitrate and potas- 
sium bromide the reaction is represented by the following equation: 


AgNO; + KBr — AgBr + KNOs3. 


Silver Potassium Silver Potassium 
nitrate bromide bromide nitrate 


7. The process of digestion; by standing from 10-20 hours at 
ordinary temperatures or by boiling, or treatment with ammonia, in 
the case of gelatine emulsions. 

8. Washing, combined with shredding, to completely remove the 
last traces of soluble salts. 

Usually, in the case of gelatine emulsions, the silver bromide is 
formed in only a portion of the gelatine, the remainder being added 
immediately after digestion. In this way the danger of destroying the 
setting power of the gelatine by heat is avoided. 

A second method of preparing emulsions consisting in the addition 
of washed silver bromide to a solution of gelatine was introduced by 
Abney. The order of the operations in this case is as follows: 

1. The soluble halide is dissolved in water. 

2. The silver nitrate is dissolved in water. 

3. The two solutions are mixed in the dark, thus producing silver 
bromide according to the reaction previously given. 

4. The mixed solution is allowed to stand from I to 4 hours in 
order that the silver bromide may precipitate. 

5. The silver bromide crystals are then washed until chemical 
tests show that there is no trace of soluble salts remaining. 


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THE PHOTOGRAPHIC EMULSION 151 


6. The washed silver bromide is added to the solution of gelatine. 

7. The emulsified silver halide is digested by boiling or by treat- 
ment with ammonia. 

Gelatine.—Gelatine belongs to that class of substances known as col- 
loids from the Greek xoAXa meaning glue. The substances of this 
class were termed colloids by a Glasgow chemist, Graham, who found 
that certain substances in solution such as albumen, glue and gelatine 
do not pass through an animal membrane, while solutions of crystal- 
line substances such as common salt do. To the former class of sub- 
stances Graham applied the term colloids; to the latter class crystal- 
loids. In colloidal solutions the subdivision of the particles is not so 
high as in the case of the crystalloids and it is for this reason that 
they do not pass through filter materials and membranes. Two other 
terms, sol and gel, were also introduced by Graham. To the liquid 
solution of a colloid he applied the term sol; to the jelly the term gel. 

The value of gelatine for photographic emulsions is due to its 
unique physical properties as well as its chemical composition. The 
easy reversibility of the transition from the sol to the gel and vice 
versa, Or 


Hydrosol > Hydrogel, 


is of paramount importance for photographic purposes and it is in 
this respect that gelatine is distinctly superior to any other colloid. 
Gelatine swells in cold water but does not dissolve. Hot water dis- 
solves it, but on cooling it again forms a jelly even if the concentra- 
tion of the solution is as low as I per cent. The formation of the 
jelly from the sol is termed setting and the reverse reaction melting 
and the temperatures at which the change of state takes place as 
setiing points and melting points. Technical gelatines are broadly 
classified as hard, medium and soft. A hard gelatine solidifies quickly 
and becomes quite hard, offering considerable resistance to reswelling. 
A soft gelatine is exactly opposite in character, solidifying slowly and 
reswelling quite easily. For emulsions a hard gelatine is easier to 
work, especially in summer or in hot climates, as the emulsion ad- 
heres to the support better and does not soften unduly in development. 
Hard gelatine, however, fogs readily and develops slowly, owing to 
the fact that the penetration of the film by the developing solution 1s 
more difficult. Accordingly, in practice the emulsion maker uses 
a medium gelatine, combining hard and soft gelatines in the propor- 


152 PHOTOGRAPHY 


tions which his experience has taught him to be the best for general 
purposes. 

Aside from acting as an emulsifying medium, gelatine acts as a 
protective colloid. If silver bromide is formed by the combination 
of solutions of silver nitrate and potassium bromide, using a slight 
excess of the latter, and the precipitated silver bromide washed to 
remove all traces of extraneous salts it will be found that on the 
application of a developer the silver bromide will be immediately re- 
duced whether exposed to light or not. Sheppard and Mees attrib- 
ute the protective action of gelatine to the insulation of the nuclei 
of the silver bromide grain with which effect is associated a delay in 
the aggregation of the silver amicrons to form larger nuclei.? 

Authorities have largely been at a loss to account satisfactorily for 
the fact that emulsions of much higher speed may be prepared with 
gelatine than with any other colloid. The earlier conception of the 
value of gelatine being due to its functioning as a photochemical 
sensitizer by absorption of halogen has largely been abandoned. Until 
only a few months ago it was a disputed point as to whether gelatine 
should be regarded as being directly responsible for the high sensi- 


tiveness of our modern sensitive materials or whether it acted merely 


as a passive medium facilitating the growth of the most sensitive 
form of the silver halide grain. Another disturbing factor was the 
action of various gelatines on emulsion sensitiveness. From the 
earliest days of gelatino-bromide emulsion it had been known that 
emulsions prepared in precisely the same manner but with different 
samples of gelatine might vary greatly in light sensitiveness. After 
methods of determining the size-frequency distribution of grains of 
silver halide in emulsions had been evolved it was possible to show 
that emulsions having the same physical characteristics as regards 
size of grain and size-distribution of grains might vary considerably 
in light sensitiveness. A long series of investigations in the Eastman 
Research Laboratory brought to light the existence of what is termed 
Gelatine-X, the presence of which in ordinary gelatine is largely re- 
sponsible for photographic sensitiveness. This Gelatine-X has been 
found to be analogous to ally] mustard oil and to be an allyl isothio- 


1 For an interesting discussion of this subject see “ Note on the Function of 
Gelatine in Development,” by Dr. T. Slater Price. Phot. J., 1925, 65,94. 


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THE PHOTOGRAPHIC EMULSION 153 


cyanate (C;H;-NCS) which reacts readily with ammonia to produce 
a thiocarbamide, e.g. 


/NHR 
R-N:C:S NH; C—S 
\NH2 
Experiments show that the group 
_ -_N—— 
C=S 
pes ich te 


is of fundamental importance for photographic sensitizing. Photo- 
graphically ? active gelatine contains only from I part per 1,000,000 
to I per 300,000 of the sensitizing substance and the presence of such 
an exceedingly small amount in a complex, many-sided substance like 
gelatine accounts for the fact that after fifty years we are just dis- 
covering the reason for reactions which have been observed since the 
earliest days of gelatine emulsions. 

Light Sensitiveness of Silver Salts.—The principal halide used in 
photographic emulsions is the bromide, which is considerably superior 
in sensitiveness to any of the other halides. The following table gives 
the comparative sensitiveness of some of the more sensitive silver 
salts. 


Sensitiveness with 


Name i Chemical Formula Sensitiveness aoa rnin’ 
Silvervoxalat@:. 4.6... AgeC204 80 
SC ea AgNO; 8 
mirver tartrate. < ov... AgeC.H.O¢ 17 
Silver. citrate. ..6....... AgsC3H;O; 18 
Silver chloride.......... Ag 100 (more intense) 
Silver todide.......:... Agl 450 


Silver bromide......... AgBr 900 


In 1874 Stas, the Belgian chemist, pointed out that silver bromide 


may exist in six molecular states, namely: * 


. A white flaky state. 

. A yellow flaky state. : 

. An intense yellow powder. 

. A pearly-white powder. 

. A yellowish-white granular state. 
. An intense yellow crystal state. 


Am BW DND #4 


2 Sheppard, Phot. J., 1925, 65, 380. 
8 Annales de Chimie, vol. ITI, p. 280. 


154 PHOTOGRAPHY 


The first and second forms are produced when aqueous solutions 
of a silver salt and a soluble bromide are mixed in the cold. These 
two forms are converted into the third or fourth state by shaking up 
well in water. If either the third or fourth modification is poured 
into boiling water it is instantly changed to the granular state of No. 5. 
Under the influence of long boiling the dull granular bromide is trans- 
formed to the crystal state which is the most sensitive member of the 
series. 

In photographic emulsions we are concerned entirely with the 
crystal state of silver bromide and the processes of emulsion manu- 
facture are directed to the production of these crystals of silver 
bromide in gelatine in such a way as to obtain the highest possible 
sensitiveness consistent with the other properties required for satis- 
factory emulsions. 

The Preparation of Emulsions.—The student will have perceived by 
this time that emulsion making is a very exacting and complex process 
which demands, not only a thorough training in the chemistry of col- 
loids and in physical chemistry, but also a-large amount of applied 
knowledge with respect to the operations of emulsion making which 
can only be gained from actual experience. Our knowledge of the 
fundamental principles involved in the processes of emulsion making 
are still unsatisfactory and the subject is in fact more of an art than a 
science. What we know about the preparation of emulsions and the 
influence of various factors on the properties of the finished emulsion © 
has been gained entirely by empirical experimentation extending over 
a long range of years. While constant experimenting has shown the 


emulsion maker the conditions essential to the preparation of emul- 


sions of high sensitiveness, we know but little of the fundamental 
causes involved, the ultimate differences which we find from one 
emulsion to another, and between different particles of the same 
emulsion. Work on some of these problems is being conducted at 
the present time and it is probable that some of these points may be 
cleared up in the near future. Owing to their commercial value it is 
difficult to say to what extent these matters will become common 
knowledge. or 
While in discussing the preparation of gelatine emulsions it will 
- be necessary to divide the subject into three heads—emulsification, 
ripening or digestion, and washing—it should not be assumed that 
these operations are entirely separate and distinct and independent of 


ee ee ee Oe ee 


THE PHOTOGRAPHIC EMULSION 155 


each other, but on the contrary that they are closely related and 
mutually interdependent upon one another. The sensitiveness of the 
silver halide grain is influenced by practically every feature of its en- 
vironment from the time of its emulsification to coating. The con- 
centration and proportions of the various substances, the temperature 
at which the various operations are conducted, the character of the 
gelatine used, the alkalinity or acidity of the emulsion during digestion 
and the time occupied in the various operations, all influence the sen- 
sitiveness and character of the emulsion to a marked degree. Thus 
if emulsification has not been conducted under conditions which are 
favorable to the formation of relatively large grains of silver halide 
as well as the proper proportion of the various sized grains, no man- 
ner of digestion will produce a highly sensitive emulsion. In other 
words, it is not possible to convert a low speed emulsion into one of 
high speed simply by digestion; if an emulsion of high sensitiveness 
is determined upon, it must be borne in mind from the beginning and 
conditions provided which are favorable to the formation of the most 
sensitive grains of silver halide. Hence while for purposes of dis- 
cussion the various operations will be treated separately, it is to be 
understood that in reality they are closely related to one another and 
not separate and distinct as the manner of treatment might indicate. 

Emulsification.—When silver nitrate and potassium bromide are 
mixed in the presence of gelatine it is usual to use an excess of the 
latter salt. In the presence of gelatine, free silver nitrate is easily 
decomposed during the process of digestion and the emulsion fogs 
on development. In theory, it should be possible to use equivalent 
amounts of silver salt and soluble bromide so that neither would be 
in excess, but it is not possible in practice and therefore it is usual to 
use an excess of soluble bromide. The proper proportion between the 
two is a matter of dispute. Dr. J. M. Eder, the celebrated Austrian 
authority who took a very active part in the development of gelatine 
emulsions, favored a proportion of 5-4. Sir William Abney, the 
eminent English investigator, favored a ratio of 15-11; while Ben- 
nett and Wilson advised 11-7 when using ammonium bromide, and 
W. K. Burton 42-25. } 

The use of an excess of soluble bromide has certain positive ad- 
vantages. When a considerable excess of soluble bromide is used 
there is less danger of fog when the emulsion is digested by heating 
for a long time, or by the addition of ammonia. The use of an ex- 


156 PHOTOGRAPHY 


cess of soluble bromide enables the process of digestion to be carried 
further without producing an emulsion which fogs upon being placed 
in the developer and thus enables a higher degree of sensitiveness 
to be obtained. Abney also showed that a considerable excess of 
soluble bromide gives a more sensitive emulsion, when digested by 
boiling, than one in which the silver salt and soluble bromide are 
more nearly equivalent.* 

A great excess of soluble bromide tends to produce an emulsion 
which fogs readily if it is digested for a one time or at very. ec 
temperatures. 

In making rapid gelatine emulsions a concentrated solution of silver 
nitrate is added to a solution of soluble bromide of similar concen- 
tration in the presence of gelatine and an excess of soluble bromide. 
The slight clouding which appears might lead one to assume that the 
two do not react immediately to form silver bromide but exist sepa- 
rately for some time. This is not so, for it has been found that silver 
nitrate and a soluble bromide react at once, even in the presence of 
gelatine, according to the equation 


AgNO; + KBr — AgBr + KNOs3. 


If the emulsion is examined with a microscope from time to time 


as additional silver solution is added, a gradual growth in the number 
of silver halide grains is observed. At the same time, it will also be 


observed that the grains already formed are increased in size, show- - 


ing that not all of the silver solution added goes to form new grains 
of silver halide but that some is added to those already existing. Con- 
sequently, the grains of silver halide grow, not only in number, but 


also in size, with the addition of the solution of silver nitrate. The 


size of the silver halide grain formed depends upon the amount of 
free potassium bromide present, the temperature and the concentra- 
tion and rate of addition of the silver solution. | : 

The freshly precipitated emulsion, especially if emulsified at a low 
temperature and under conditions favoring the formation of very small 


— ee ee 


particles of silver halide, is very fine grained and relatively transparent 


but is only slightly sensitive. When digested by heat or ammonia its 
speed increases from 100 to 1000 times.® 

4 Photo. News, 1881 p. 108. 

5 See Eder, Handbuch der Photograpme, 1, 24 (Ed. 1902). 

Despite some advantages, Abney’s method of emulsifying washed silver bro- 
mide in gelatine is not generally followed and so far as the writer has been 


alee ge heel 


—Je”6hUL 


—— se oe 


THE PHOTOGRAPHIC EMULSION 157 


Gelatino-Bromo-Iodide Emulsions.— Notwithstanding the fact that 
gelatino-iodide emulsion is only slightly light sensitive, a gelatino- 
bromo-iodide emulsion containing a small amount of iodide is more 
sensitive than one consisting of pure silver bromide. The iodide does 
not crystallize out separately but enters into the structure of the crystal 
of silver bromide in some way as yet undetermined. According to 
Eder © the addition of iodide to gelatino-bromide emulsions was first 
advised by Perry in the Yearbook of Photography for 1878. The 
effect of adding iodide was carefully studied by Abney in 1880.7 The 
addition of a small amount of iodide (generally not more than 3-7 
per cent of the total amount of soluble halide present) has several ad- 
vantages. It keeps the emulsion clear and makes a higher degree of 
digestion possible without danger of fog. Emulsions containing a 
small amount of iodide give brighter, clearer. images than those of pure 
silver bromide as well as greater contrast and density. In Germany 
and Austria nearly pure silver bromide emulsions are favored, while 
in England bromo-iodide emulsions are in greater favor. A certain 
percentage of iodide is invaluable in modern practice in the preparation 
of extra sensitive emulsions. 

Digestion of the Emulsion.—Digestion, or ripening as it is some- 
times called, is the term applied to the treatment of emulsions by heat 
or ammonia for the purpose of increasing their sensitiveness to light. 

The fact that with gelatine emulsion the sensitiveness to light in- 
creases to a certain extent simply by allowing the emulsion to stand at 
ordinary temperatures was observed by Monkhoven of Ghent in 1880.8 


A method of digestion based upon this principle was worked out by 


Cotesworth and is described by Sir William Abney in his Photography 
with Emulsions. Abney himself pointed out that a heated and already 
sensitive emulsion showed an increase in sensitiveness after being al- 
lowed to stand for a day at ordinary temperature. The increase was 
still more marked after the second or third day.° | 

If an emulsion is melted and allowed to solidify it is found that the 


able to ascertain it has never been used in the manufacture of emulsions on a 
commercial scale. Full details of this method may be found in Photography 
with Emulsions by Sir William Abney, while the matter has been discussed by 
Dr. Liippo-Cramer at a later date in a lengthy paper in Photographische Kor- 
respondenz, 1907, 44, 572. 

6 Handbuch der Photographie, I, 117-121 (5 Ed. 1902). 

7 Photo. News, 1880, 174, 196. 

8 Phot. Archiv, 1880, p. 197. 

9 Photo. News, 1880, p. 567. 


12 


158 PHOTOGRAPHY 


sensitiveness is increased considerably. The increase is still more 
noticeable if the emulsion is melted a second time. 

If the emulsion is allowed to stand several days in a water bath at 
30-40 degrees Centigrade, the sensitiveness increases from two to as 
much as ten times. This is essentially the method introduced by 
Bennett in 1878 but has been superseded by later methods because the 
long heating affects the setting power of the gelatine. If the tempera- 
ture is raised to 60 degrees Centigrade, digestion is complete in two or 
three hours, while if the temperature of the emulsion is raised to 100 
degrees digestion proceeds rapidly and is complete within twenty 
minutes to one half hour. 

Johnson advised the addition of ammonia to the emulsion in the 
British Journal Almanac for 1877 and two years later Monkhoven 
showed that digestion might be effected without the aid of heat by the 
application of ammonia to the emulsion. The next year (1879) Monk- 
hoven pointed out the great increase in sensitiveness secured by the 
addition of ammonia to an emulsion partly digested by heat.1® Con- 
siderable difficulty was experienced in the use of ammonia and it was 
not until after the investigations of Eder in 188011 that ammonia was 
used with much success. To-day ammonia is universally used in the 
preparation of ultra rapid emulsions. The advantages of ammonium 
carbonate and sodium carbonate in place of ammonia were pointed out 
by Eder in the paper above referred to, but neither is quite as effective 
as ammonia, the degree of sensitiveness secured being lower than that 
obtainable by the use of ammonia. 

Only a very dilute solution of ammonia is required; in fact more 
than 5 per cent concentrated ammonia tends to produce an exceedingly 
coarse grain which is visible to the eye in extreme cases, while at the 
same time the gelatine is attacked and its setting power largely de- 
stroyed. An excess of ammonia also causes the emulsion to fog upon 
the application of an energetic developing solution. The concentration 
of ammonia which may be used with safety depends largely upon the 
temperature of digestion, the type of gelatine employed and also to a 
certain extent on the conditions of precipitation. 

Ammonia may be added shortly after emulsification and the entire 
operation of digestion carried out at ordinary temperature, or the 
emulsion may be partly digested by heat and the ammonia added after 
the emulsion has cooled down to about 30 degrees Centigrade. The 

10 Phot. Korr., 1879, 16, 197. 

11 Sitzungsber. Akad. Wiss. Wien (1880), 81, II, 687. 


‘ 
: 
; 
; 
{ 
4 


THE PHOTOGRAPHIC EMULSION 159 


latter method gives the most sensitive emulsions and is probably that 
used in preparing the ultra rapid plates of commerce. It appears to 
have originated with Monkhoven in 1879. 

Fog.—Over digestion produces a coarse granular emulsion, the 
particles of which are visible to the eye in some cases, while the plate 
fogs in the developer, blackening whether exposed to light or not. All 
methods of obtaining an extremely sensitive emulsion lead to plates 
which fog in development. When digested by heat, fog appears 
earlier in neutral emulsions than in those which are slightly acid and 
for this reason a trace of acid is sometimes added to make sure that 
the emulsion is in a slightly acid state. Too much acid, however, is 
harmful as it delays digestion and affects the gelatine. Alkaline emul- 
sions are completely digested at low temperatures and tend to produce 
fog if digestion for a high degree of sensitiveness is attempted. The 
addition of iodide to emulsions tends to prevent fog, as does the pres- 
ence of an excess of soluble halide, while, as already mentioned, the 
addition of a trace of acid to emulsions which are digested by means 
of heat materially reduces the danger of fog. The danger of excessive 
fog is of course much higher with high speed emulsions than with low: 
in fact a certain amount of fog is inseparable from an extremely sensi- 
tive emulsion, but the amount is‘so small, under favorable conditions of 
manufacture, as to be of little consequence. 

Theory of Digestion—There is a progressive growth in the size of 
the crystals of silver halide during the process of digestion. The 
smaller grains disappear, combining with the larger grains until the 
largest crystals reach a diameter at times equal to 8 microns. It has 
long been supposed that the increase in sensitiveness due to digestion 
is connected with, if not directly due to, the growth in the size of the 
crystals of silver halide, for it is fairly well established that the larger 
crystals are generally more sensitive than the smaller. While some 
connection undoubtedly exists, recent investigation by Svedberg and 
Anderson, Renwick, Trivelli and Sheppard and others seems to indi- 
cate that the relationship. between size and sensitiveness may be con- 
sidered to be purely incidental, and in no way directly connected, since 
it is possible to prepare a low speed emulsion having grains which are 
appreciably coarser than those of most ultra sensitive emulsions. 

The principal theories of the digestive process are four in number: 

t. A molecular change occurs which results in a more sensitive form 
of silver halide. 

2. There is a partial reduction of the silver halide to silver sub- 
halide so that the light has less work to do. 


160 PHOTOGRAPHY 


3. A gelatino-silver halide complex is formed. 

4. A photocatalysis consisting of colloid silver is formed. 

The process of digestion is explained by Eder in his Handbuch der 
Photographie * as being due to the production of the most sensitive 
form of silver bromide according to the researches of Stas (page 
153). This theory is satisfactory so far as it goes, but it hardly goes 
far enough to be of any real value. 

Luther, on the other hand, believes that we have to deal with a 
change similar to that produced by light. Gelatine being an organic 
substance acts as a reducing agent and reduces the silver halide to sub- 
halide. The presence of silver sub-halide forms a part of the work 
which the light must do, consequently less exposure is required.** 

The sensitiveness of emulsions of silicic acid, which is not an 
organic substance, is against Luther’s theory as is the fact that gela- 
tine-free halide is susceptible of digestion.1* Furthermore, that di- 
gestion does not necessarily mean a reduction of silver halide to sub- 
halide is shown by the fact that it is possible to use over the silver 
bromide from a spoiled emulsion if it is re-emulsified in fresh gela- 
tine. The acceptance of the theory also implies the belief in the sub- 
halide theory of the latent image which many, if not most, investi- - 
gators now discredit.?® 

The existence of a gelatino-silver halide complex of undefined com- 
position and state as a factor in digestion has been brought forward by 
a number of investigators. While something of the kind possibly 
exists, there is at present little evidence to show that it alone is re- 
sponsible for the increase in sensitiveness produced by heating or treat- 
ment of emulsions with ammonia. 

There is a growing tendency among the men who are in intimate 
touch with the various phases of emulsion manufacture to consider that 
in dealing with photographic emulsions we are not dealing with pure 
silver halide in gelatine, or a mixture of these, but also with a minute 
trace of another substance which is supposed to be colloidal silver. 
These traces of colloidal silver are supposed to be formed as a result 
of the digestion process, and it is considered by some that the growth 
of sensitiveness progresses as the amount of colloid silver increases, 


1411137, £1800). 

18 Die Chemischen Vorgange in der Photographie, 57 (1809). 

14 Schaum and Braun, see Mees and Sheppard, Investigations, p. 267. 

15 See also Luppo-Cramber, Phot. Korr., 1904, 41, p. 164, for some pertinent 
remarks regarding this theory. 


oe ee 


_— 17s: 


a ore 6 ee re oe eee eS eee ee eee: el. 


THE PHOTOGRAPHIC EMULSION 161 


until the point is reached where the excess of colloid silver is so great 
as to render the emulsion immediately reducible on application of the 
developer, whether exposed to light or not. Luppo-Cramer in nu- 
merous papers *® and Renwick ** in his Hurter Memorial lecture of 
1920 supported colloid silver theories. 

There are several points in favor of a colloid silver theory. Factors, 
such as the presence of an alkali or increase in temperature, which 
would tend to cause partial recrystallization and the formation of col- 
loid silver by reduction of the silver halide all facilitate the “ ripening ”’ 
of emulsions and increase the sensitiveness of the grain. Moreover it 
has been shown that colloid silver may be light sensitive to a very high 
degree. 

The recent work of Sheppard on the sensitivity promoting substance 
in gelatine, however, while it does not exclude the possibility of colloid 
silver formation as a factor in digestion, indicates that ‘‘ ripening” is 
concerned primarily with the conversion of the allyl isothiocyanate 
found in photographically active gelatines into allyl thiocarbamide, 
which reacts with the silver halide grain in such a way as to greatly 
increase its sensitiveness to light. 7 

Eliminating the Soluble Salts.—After the extra gelatine has been 
added to the digested emulsion, it is well shaken up and then poured 
out into a porcelain tray and allowed to set. The time required for 
setting will vary according to the type of gelatine used, the temperature 
and also the humidity of the surrounding air. Two hours is generally 
sufficient and often very much less is required. When the emulsion 
has set it is ready for washing to remove the soluble salts. 

In dealing with small quantities the emulsion is gathered up in a 
canvas bag which is placed under the surface of clean cold water and 
by gentle pressure the emulsion is forced through the interstices of 
the canvas. For this purpose the canvas should be as coarse as pos- 
sible. A mesh of about 8 lines to the inch is sufficiently fine. This 
divides the emulsion up into fine shreds and enables the soluble salts 
to quickly pass out in running water. Generally the operation is re- 
peated once or twice and the gelatine left in running water for one or 
two hours. 

In dealing with large quantities, as on a commercial scale, the emul- 


sion is placed in a press similar to that shown in Fig. 120. This con- 


sists essentially of a piston and a cylinder having a bottom of a netting 


16 Kolloidchemie und Photographie, 1920, 2d Ed. 
17 J, Soc. Chem. Ind., 1920, 156T, 39; Brit. J. Phot., 1920, 67, 447, 463. 


1 > ae 
ae = 


162 PHOTOGRAPHY 


of silver wires. The pressure applied forces the emulsion through the 
interstices of the wire screen into the running water below, where, 
owing to its fine state of division, the soluble salts are rapidly removed. 


(From Eder’s Ausfiihrliches Handbuch) (From Eder’s Ausfiihrliches Handbuch) 
Fic. 120. Emulsion Washing Apparatus Fic. 121. Centrifugal Separator 


Another method sometimes employed consists in the use of a spe- 
cially made centrifugal separator which is similar in general principle 
to the cream separator used in dairy manufacture. A separator as 
used for separating the soluble salts from photographic emulsion is 
shown in Fig. 121. The liquid emulsion is placed in the vessel, 4, in 
the darkroom and the top screwed on. The vessel containing the 
heated emulsion may now be brought out into daylight and placed on 
the spindle, S. The crank is then turned for several minutes, the 
actual rate of rotation being about 4000 revolutions per minute. 
Finally the vessel A is removed and taken to the darkroom, the top re- 
moved and the liquid emulsion poured off. The solid salts extracted 
from the emulsion collect on the sides of the vessel and may be re- 
moved and re-emulsified in a fresh lot of gelatine. In using a centrif- 
ugal separator the soluble salts are removed before the remainder of 
the gelatine is added: when removed by washing, the gelatine is added 
first. 

The emulsion may now be regarded as complete, but it is customary 
to add a small amount of chrome alum in order to harden the gelatine 
slightly so that it will adhere to the plate in coating and also remain 
firm during development, fixing, etc. Since our purpose in this chap- 
ter is to discuss the subject of emulsions from a theoretical standpoint 
and not with the idea of enabling the student to prepare his own plates, 


THE PHOTOGRAPHIC EMULSION 163 


the operations of coating, drying and packing will be omitted. For 
information on these points reference should be made to larger and 
more comprehensive works on the subject. 

The Silver Bromide Grain of Photographic Emulsions.—When ex- 
amined under a high power microscope, the photographic emulsion is 
seen to consist of numerous semi-transparent and practically opaque 
grains of silver halide imbedded in gelatine. These grains of silver 
halide are definitely crystalline (Fig. 122) and of various forms and 


Fig, 122. The Photographic Emulsion under a Microscope 


sizes; the most constantly recurring forms being triangles and hexa- 
gons, some of which are irregular, while all have rounded corners, but 
occasionally a long rod-shaped crystal is observed. The grains also 
vary in transparency, some being almost completely transparent while 
others are nearly opaque. Since the opaque grains behave in exactly 
the same way as the transparent grains, there is no justification for 
assuming that they are different substances. In addition to these there 
are ultra-microscopic grains which are beyond the limit of visibility 
with the highest power of the microscope. Recent investigation has 
shown that these are also crystalline and have substantially the same 
structure as those of larger dimensions.1* There is no evidence for 
the existence of non-crystalline silver bromide in photographic emul- 
sions. 3 

The size of the silver halide grains in commercial emulsions ranges 
from the ultra-microscopic particles of less than one micron to grains 


18 Wilsey, Phil. Mag. (1922), 42, p. 262. 


164 PHOTOGRAPHY 


as large as 3 or 4 microns. In positive emulsions the larger number 
of grains are either ultra-microscopic or very small, while in the case 
of highly sensitive negative emulsions, although a large number of 
ultra-microscopic grains are present, the majority of the grains are of 
microscopic size, while all are of course definitely crystalline. 

From Fig. 122 it might be assumed that all the halide grains com- 
posing the emulsion are to be found in one layer. This is not so; 
there are a number of layers, sometimes as many as ten or twelve, de- 
pending somewhat upon the character of the emulsion. This is indi- 
cated in Fig. 123, which is a photomicrograph of a cross-section of a 


(Courtesy of Dr. A. P. H. Trivelii.) 
Fic. 123. Cross Section of a Developed Emulsion 


developed portrait film by Dr. A. P. H. Trivelli of the Eastman Re- 
search Laboratory.1® The number of grains in a given area of a 
coated plate is enormous. ‘The number varies with the type of emul- 
sion but averages from 10 to 25 billion per square inch.”° 


The Sensitivity of the Silver Halide Grain.— Microscopical investi- 


19 Reproduced by permission of Dr. Trivelli. . : 
20K. P. Wightman, Amer. Phot.: (1923), 17, Pp. 329. 


Se | ee 


| 
: 
F 
| 


THE PHOTOGRAPHIC EMULSION oELOG 


gation has shown that in spite of the enormous number of grains of 
silver halide and their close proximity to one another, each individual 
erain affected by light acts as a single unit and there is no trans- 
ference of development from one grain to another, unless the two are 
grouped together in absolute contact; a state of affairs characteristic 
of some emulsions.” It has also been found that a grain is either 
made developable by a certain amount of light or it is not developable. 
Thus, we do not get partial development for a certain exposure fol- 
lowed by more for a longer exposure but up to a certain amount of 
light action the grain is undevelopable and after that amount is 


reached it is rendered completely developable. The amount of light 


required to make a grain developable represents what is termed the 
sensitivity of the grain. 

Investigation of the number of grains made developable by a given 
exposure shows that all the grains are not equally sensitive; because 
under such conditions all the grains would become developable as soon 
as the exposure reached a certain value. Microscopical examination 
at high powers shows that the grains of silver halide differ widely in 
size and on counting the number of grains made developable in given 
size-classes, it is found that in one and the same emulsion the sensi- 
tivity increases with the size of the grain. This does not necessarily 
mean that all large grains are more sensitive than smaller ones, for 
with different emulsions the reverse is often true,?* but if we keep to 
the same emulsion the larger grains are on the average more sensitive 
than the smaller. There are, however, some differences in sensitivity 
among grains of the same size and shape and from the same emulsion. 

Hodgson ** in 1915 showed that the developer attacked the grain 
of silver halide at certain preferred points (Fig. 124) and Svedberg, 
who investigated the subject more thoroughly (for list of papers see 
bibliography), found that these centers were scattered in a haphazard 
fashion among the grains and that the larger the grain the more likely 
it was to have a developable center. He also pointed out that the dif- 
ferences in the sensitivity of grains of one size was in agreement with 
the chance distribution of development centers. Svedberg was of the 
opinion that these development centers were due to the discrete nature 

21 Svedberg, Phot. J., 1922, 62, 183. Slade and Higson, Proc. Roy. Soc., 1920, 
A 98, 154. ‘Trivelli, Righter and Sheppard, Phot. J., 1922, 62, 407. Trivelli, 
Brit. J. Phot., 1922, 69, 687. 


22 Sheppard, Phot. J., 1921, 51, 400. Renwick, Phot. J., 1921, 51, 333. 
23 Jour. Franklin Inst., 1917, 184, 705; Brit. J. Phot., 1917, 64, 654. 


166 PHOTOGRAPHY 


of light emission and that the necessary and sufficient condition for 
the formation of a developable center was that a certain number of 
light quanta fall upon the grain within a certain minimum area. Sil- 
berstein, like Svedberg, was of the opinion that these facts might be — 


Fic. 124. Hodgson’s Pretend Points 


explained by means of the quantum theory of light emission; the 
variability of sensitiveness among grains of one size and the increase 
of sensitivity with the size of grain being due to the fact that it is 
only necessary for a grain to be struck by a light-dart in order to 
make it developable. 

According to this theory the development centers do not exist be- 
fore exposure and all the grains are of the same kind and substance 
just the same as if they were fragments of a large crystal; the dif- 
ferences in sensitivity being due to the fact that the larger the grain 
the more likely it is to be hit by a light-dart. 

Others, however, among them Sheppard, Clark and Toy were of 
the opinion that these development centers were present before ex- 
posure and were due to the presence of a specific chemical substance 
other than silver halide and to be largely responsible for the high 
sensitivity of the grains of modern emulsions. 

One of the important supports of this theory is the fact that a plate 


THE PHOTOGRAPHIC EMULSION 167 


can be largely desensitized by treatment with an oxidizing substance, 
such as sodium arsenite or chromic acid, which are afterwards re- 
moved from the emulsion before exposure. Investigation has shown 
that chromic acid destroys the latent image and greatly reduces sen- 
sitivity but that the effect on the former is much more pronounced 
than on the latter. Hence it appears that the sensitivity centers exist 
before exposure and are not identical with the latent image. Clark 
was able to show that pre-exposure to light greatly increased the de- 
gree of reduction in sensitivity on treatment with chromic acid while 
Wightman and Sheppard showed that in the same emulsion the 
smaller grains are relatively reduced more in sensitivity than the 
larger ones. 

Accordingly: (1) the conversion of the sensitivity substance of the 
development centers into latent image substance facilitates the attack 
of oxidizing agents ; (2) the sensitivity substance is held in a different 
way in the larger grains; or (3) the conversion of sensitivity sub- 
stance to latent image substance increases the probability of develop- 
ment for the larger grains. 

The evidence, therefore, while it does not exclude the possibility of 
a discrete light action as suggested by Svedberg and Silberstein, does 
support the theory that there is present in the silver halide grain a 
substance other than silver bromide which increases grain sensitivity. 
The discovery of the sensitizing substance in gelatine by Sheppard 
still further supports the claims of those who believe in the existence 
of a foreign substance in the grain, the presence of which in varying 
amounts or states is responsible for the variation of sensitivity of 
grains of different sizes and for the existence of the development 
centers. 

The Nature of the Sensitivity Substance——Those conversant with 
the subject of emulsions have long been of the opinion that in the 
preparation of gelatino-silver halide emulsions a slight reduction of 
silver halide takes place; resulting in the formation of a sub-halide or 
colloid silver. Luppo-Cramer ** as well as Renwick *° have supported 
the hypothesis that the sensitivity substance is colloid silver of high 
dispersity. Walter Clark on the other hand considered the sensitivity 
substance to be either silver oxide or hydroxide due to the absorption 


24 Luppo-Cramer, Kolloidchemie und Photographie, 2d Ed., 10921. 
25 Renwick, J. Soc. Chem. Ind., 1920, 156 T, 39; Brit. J. Phot., 1920, 67. 


168 : (PHOTOGRARI 


of OH ions.2® This theory was based upon the work of Fajan and 

Frankenburger *” on visible decomposition. | 
However, as a result of the work of Sheppard in the laboratories 

of the Eastman Kodak Company it has been well established that the 


substance composing the sensitivity centers is silver sulphide. In- 


vestigation of the causes of variation in the sensitiveness of emulsions 
of similar properties but different gelatines showed that this variation 
was due to the presence of a sensitivity promoting substance which 
was identified as allyl isothiocyanate. This reacts with ammonia to 
form allyl thiocarbamide which unites with a small amount of the 
silver halide to form a double compound which at higher temperatures 
and in alkaline solution decomposes to form silver sulphide.”* 

The exact manner in which the minute specks of silver sulphide are 
held in the grain of silver halide has not yet been definitely determined. 
It seems, however, that since the crystal lattice of the silver sulphide 
interpenetrates with that of silver bromide it is possible that the ad- 
hesion of the silver sulphide speck to the silver halide grain may vary 
in firmness and it is conceivable that this might affect its sensitizing 
power. 

The relation of the sensitivity centers of silver sulphide to the mech- 
anism of the formation of the latent image will be left until the next 
chapter. | 

Grain-Size Distribution and its Relation to the Photographic Prop- 
erties of Emulsions.—Investigation having shown that the individual 
halide grain is the photochemical unit of the photographic plate, the 
properties of the emulsion representing simply the sum of the prop- 
erties of the individual grains modified by their positions in layers, a 
study of the effect of grain-size distribution in emulsions and its rela- 
tion to photographic properties is of great importance. For if emul- 
sion sensitiveness is merely a matter of grain-size distribution the 
emulsion maker has only to provide the conditions favorable to the 
growth of crystals of the proper size in order to produce emulsions of 
the highest possible sensitiveness or having any other required prop- 
erties. On the other hand, should it be shown that photographic 
properties are not wholly, or only partially, controlled by grain-size 
distribution but by other factors as well, the line of investigation must 
naturally be directed along entirely different lines. . 


26 Clark, Brit. J. Phot., 1923, 70. 


27 Fajan and Frankenburger, Zeitischr. Physik. Chem., 1923, 105, 255, 273, 320. - 


28 Sheppard, Phot. J., 1925, 65. 


THE PHOTOGRAPHIC EMULSION 169 


As early as 1895 Gaedicke ®® called attention to the probability of 
some relation between the size of grains and the sensitiveness of an 
emulsion and Mees in 1915 *° suggested that “ inasmuch as emulsions 
are not homogeneous, but contain grains of all sizes, the sensitiveness 
of the emulsion will depend upon the distribution of the different 
sizes of grains, as will also the shape of the characteristic curve.” ** 
Slade and Higson as the result of some investigations on the action of 
light on an emulsion containing grains of very nearly the same size 
and only one layer thick also concluded that the properties of the 
emulsion are determined mainly by the relation of the different sizes 
of grains to one another and the quantity of each present.*? Svedberg 
found that for every class of grains of nearly the same size in the 
emulsion there is a distinct characteristic curve.*® 

The matter was not fully investigated in a quantitative manner until 
1921 when Sheppard, Wightman and Trivelli of the Eastman Re- 


‘search Laboratory published the first of a series of papers on the 


subject (see bibliography). They attacked the problem by photo- 
micrographing the grains of various emulsions at a magnification of 
2000 times and then enlarging the negative five times, so that the 
actual magnification equalled 10,000 times. The developed grains 
of a given area were then measured and divided into classes accord- 
ing to size. The data secured in this manner may be represented 
graphically by plotting the number of grains of each class as ordinates 
against the sizes of the grains as abscisse. In Fig. 125 are shown 
photo-micrographs of the emulsion of a portrait film and a Standard 
slow lantern plate together with curves showing the size-frequency 
distribution of each. It will be observed that the grains of the posi- 
tive emulsion are all comparatively small and uniform, the range be- 
ing from about 0.2 to 1 micron. The high speed portrait film, on the 
contrary, shows an extended range of sizes from about 0.2 micron to 
as high as 2.7 microns with a maximum close to 0.5. 

A correlation of these facts and the photographic properties of 
emulsions is to be the subject of further investigation. The data 
which has been accumulated shows definitely that the relative speed 
of an emulsion increases rapidly with an increase in the average size, 

29 Eder’s Jahrbuch, 1895, p. 200. 

80 Tbid. 

81 J. Franklin Inst., 1915, 179, 141. 


82 Phot. J., 1919, 59, 260. 
88 Z, Wiss. Phot., 1920, 20, 306. 


eaeet. 


170 PHOTOGRAPHY 


and range of size, of the grains contained in the emulsion. Several 
other interesting relations have been indicated in the course of the in- 
vestigation and these points are now being investigated. At the 


80 SIZE — FREQUENCY CURVE 


A= STANDARD SLOW LANTERN SLIDE 


Le B= PAR SPEED PORTRAIT FILM 


FREQUENCY PER 1000 GRAINS 
> 
iJ 


20 


100 


250 ee 


Fic. 125, Size Frequency Distribution of Silver Halide Grains in a Portrait 
Film and Lantern Slide Emulsion 


0.5 1.0 1. 


present time all that can be definitely stated is that there is apparently 
a very close connection between grain size and size- frequency and 
the photographic properties of emulsions. 


———— 


THE PHOTOGRAPHIC EMULSION 171 


GENERAL REFERENCE WorKS 


AsNEY—Photography with Emulsions. 

AxsNEY—Treatise on Photography. 

AsneEy—Instruction in Photography. 

Burton AND PrincLE—Processes of Pure Photography. 
BrotHerS—A Manual of Photography. 

Eper—Ausfurliches Handbuch der Photographie. 

EpER AND VALENTA—Beitrage zur Photochemie. 
Luppo-CraMER—Kolloidchemie und Photographie. 
LutHER—Die Chemische Vorgange in der Photographie. 
MEES AND SHEPPARD—Theory of the Photographic Process. 
SHEPPARD AND TRIVELLI—The Silver Halide Grain of Photographic Emulsions. 
TISSANDIER—History and Handbook of Photography. 
VALENTA—Photographische Chemie und Chemikalienkunde. 


CHAPTER VII 
ORTHOCHROMATICS 


Light and Color. The Spectrum.—According to the generally ac- 
cepted theory, light is an undulatory movement in an elastic medium 
known as the ether. When this elastic medium is set in vibration, a 
wave-movement is sent out in all directions from the source at the 
speed of about 186,000 miles per second. If the vibrations are below 
or higher than a certain limit, they cannot be detected by the eye but 
may be detected in various other ways. White light consists of a 
number of wave-movements of various lengths and rate of vibration. 


When white light is passed through a prism refraction and dispersion. 


take place and the rays are sorted out into waves of different lengths 
and rate of vibration, producing what is known as the spectrum. The 
short waves are the most refrangible so that violet is refracted the 
most and red the least, while green and yellow are refracted to an 
intermediate extent and occupy a position between the violet and 


blue on one side and the orange and red on the other. The position 


of any color in the spectrum in respect to other colors is, therefore, a 
measure of its refrangibility, or the length of the ether wave. 


Although the spectrum consists of a continuous band in which the — 


colors graduate into one another, it is customary to recognize seven 
colors in the visible portion: violet, blue, green, yellow, orange and 
red. 

For purposes of reference, it is necessary to have some recognized 
means of referring to any desired portion of the spectrum. Such a 
purpose is fulfilled by the Fraunhofer lines. These are narrow dark 
lines traversing the spectrum and occurring at fixed points so that 
they form a convenient means of designation for any part of the 
spectrum. In Fig. 126 the spectrum is reproduced by the three- 
color process and the positions of the principal Fraunhofer lines are 
shown. The numbers beside the lines refer to the wave-lengths in 
Angstrom units. An Angstrom unit is equal to 1/10,000,000 of a 
millimeter and is the unit of measurement used in specifying the 
length of light waves. As we will have occasion to refer to these lines 

172 


2) 


Wave-lengths in Angstrom Units. 


7594 


7186 


6867 


6563 


5896 


5270 
5184 


4861 


4308 


3969 
3934 


7594 
7186 


6867 
6563 


5896 


5270 
5184 


4861 


4308 


3969 
3934 


1. Prismatic Spectrum. — 2. Spectrum produced by diffraction grating. 


(Showing the principal Fraunhofer lines.) 


Fic. 126.—Three-color print of the Solar Spectrum. 


° ' 
Wave-lengths in Angstrom Units. 


ORTHOCHROMATICS 173 


and wave-lengths, the student should study the three-color print care- 

fully and learn the lines and their relative positions in the spectrum. 
Visual and Photo-Chemical Luminosity.—Of the seven colors which 

form the visible spectrum, yellow is the most luminous to the eye. 


DARK VIOLET BLUE GREEN YELLOW RED DARK 
Fic. 127. Visual Luminosity of the Spectrum after Abney 


The relative visual intensities of the various colors of the spectrum 
are illustrated in Fig. 127 from Abney,’ the heights of the curve 
above the horizontal line giving the relative intensity. It will be ob- 
served that the maximum intensity is very close to the D line. On 
either side of this point the visual intensity of the colors decreases, 
the drop of the curve being especially noticeable in the blue and 
violet. 

If a sensitive plate is exposed in a spectrograph and the densities, 
which are a measure of the work accomplished by light, are plotted 
as above, we will find that the silver halides have a totally different 
sensitiveness from that of the eye and that the maximum sensitive- 
ness of the plate is found in the violet, while in the yellow near the 
D line, where the maximum visual luminosity lies, the plate is prac- 


tically insensitive. It will be still more instructive if instead of an 


ordinary plate we use the silver halides themselves. Draper, Hunt, 
Herschel and, more notably, Abney studied extensively this action of 
the spectrum on the silver halides and the latter gives the following 
curves which show the sensitiveness of the chloride, bromide and 
iodide of silver to the spectrum (Fig. 128).2 The dotted lines in- 
dicate the extension of sensitiveness resulting from extreme lengthen- 
ing of the exposure. 

1 Instruction in Photography, toth Ed., p. 6. 

2 Instruction in Photography, toth Ed., p. 9. Also see Meldola, Chemistry of 


Photography, p. 255. 
13 


174 PHOTOGRAPHY 


The result of mixing the halides is to secure slightly more sensitive- 
ness in the blue-green but in no case does the increase begin to ap- 
proach the. visual luminosity curve of the spectrum. (See Meldola, 
Chemistry of Photography, p. 208.) 

Since the visual luminosity of the spectrum is so totally different 
from the photo-chemical activity of the spectrum, it follows that an 
ordinary plate containing only the silver halides cannot reproduce 
colors in their proper relation to one another. Blue objects appear 
much lighter in photographs than they do to the eye, while yellow is 
reproduced as black. As a typical example, we may take the case 
of an orange on a blue velvet cloth. Now of the two, the orange is 
much the lighter, so much so that the blue appears dark in com- 


fi dese Sees G i ee 


Fic. 128. Spectral Sensitiveness of the Silver Halides after Meldola 


parison. When photographed, on an ordinary plate, what do we get? 
The brilliant orange is a dark grey, also black, while the blue has 
turned out almost white and, therefore, the color rendering is totally 
false. Many other examples might be given to show the false render- 
ing of color given by ordinary plates. 

. The incorrect rendering of color was for a long time a serious ob- 
stacle to the progress of photography but fortunately means have 
been found: which overcome this difficulty and there is now no dif- 
ficulty in securing proper color values if the proper materials and 
skill are used. This notable advance has been made possible through 
the discovery of the fact that certain dyes render the silver halides 
sensitive, not only to the violet and blue, but also to the green, yellow 
and red. 

History of Dye Sensitizing.—Dye sana dates from Vogel’s 
discovery of the action of corallin in 1873.* Considerable difficulty 
was experienced in sensitizing gelatine emulsions with corallin and 


3 Vogel, Handbuch der Photograpme, 4th Ed., I, p. 204. 


ORTHOCHROMATICS 175 


it was not until the discovery of eosin in 1882 that a really practical 
sensitizer for green and yellow was found. Following this encourag- 
ing discovery, many other dyes were investigated by Waterhouse, 
Vogel, Schumann, and Eder, the last named examining, together with 
his students, several hundred dyes which might be suspected to pos- 
sess sensitizing properties. Very few, however, were found which 
were of practical value and only two are in use to-day, erythrosin, a 
strong, yellow-green sensitizer, and cyanine, a fairly good orange- 
red sensitizer. In 1904-5 Konig introduced pinachrome, ortho- 
chrome T, pinacyanol and dicyanine, all of which are more efficient 
than any of the dyes previously known and are now in general use. 
Quite a number of interesting dyes for color sensitizing were dis- 
covered by Sir William Jackson Pope in 1920* while three new dyes, 
naphthacyanole, acetaminocyanole, and kryptocyanine, have been in- 
troduced by Mees and Gutekunst of the Eastman Research Labora- 
tory.° 

Known Facts Regarding Color Sensitizing.—The most valuable 
work on the theory of dye sensitizing has been done by Eder from 
which the following facts are summarized: ° 

1. The dye must stain the silver halide grain. 

2. Vigorous sensitizing dyes are substantive dyes. That is, they dye 
substances directly without a mordant. Staining of the silver halide 
grain is no proof of color sensitizing. 

3. A dye sensitizes for the rays which it absorbs or more accurately 
the rays absorbed by the dyed silver halide. 

4. The maximum of sensitiveness lies at about the same place as 
the maximum absorption of the dye, with a slight shift towards the 
red. Stated more correctly, the maximum of sensitiveness agrees with 
the maximum absorption of the dyed silver halide. 

5. A dye having a narrow band of absorption sensitizes a narrow 
band while dyes having broad bands of absorption give broad bands of 
sensitiveness. | 

6. The brilliancy of the dye appears to have no special influence. 

7. The sensitizing power of a dye does not appear to be dependent 
upon either its fugitive character or its fluorescence. 

8. No relation can be found between sensitizing power and the 
chemical composition of the dye. 

Photo. J., 1920, 60, 183, 234, 253. 

5 Brit. J. Phot., 1920, 60, 474. 


6 Ausfiihrliches Handbuch der Photographie, vol. III, p. 150. Grundlage der 
Photographie mit Gelatine-Emulsionen. 


176 PHOTOGRAPHY 


There are two methods of dye sensitizing: (1) bathing an ordinary 
blue-sensitive plate in a solution of the dye and (2) incorporating the 
dye with the emulsion. In general, greater sensitiveness results from 
the first method but plates prepared by the latter method appear to 
keep better. 

The amount of dye required is very small. The usual degree of 
concentration varies from I part in 1000 to I part in 75,000. 

It is found that in order to sensitize, a dye must combine with the 
silver halide. Whether there is chemical or molecular combination we 
do not definitely know. Eder has elaborated the latter theory,’ and 
assumes that the vibrations are absorbed by the colored compound and 
photochemical decomposition then occurs. The researches of Luppo- 
Cramer and Traube,® if they do not prove the existence of chemical 
combination between the silver halide and dye, show that there is a 
very close connection between the two. It is found that it is impos- 
sible to remove the last traces of dye from an emulsion even with re- 
peated washings. Moreover, the plate after washing still shows the 
characteristic absorption and sensitiveness of the dyed silver halide. - 

Eder’s third conclusion is practically the same as Draper’s law, which 
is the foundation of orthochromatics, and states that only those rays 
can act chemically on a body which are absorbed by it. Light which 
passes through a substance or is reflected from it cannot have any 
chemical action. 

According to Eder,® neither the maximum point of absorption of the 
dye nor the maximum point of absorption of the dye in gelatine agree 
with the maximum point of sensitiveness of the dyed silver halide. 
The maximum photographic sensitiveness lies further to the red by 
about 20 millimicrons than the maximum absorption point of the dye 
in gelatine.° That dyes having narrow intense bands of absorption 
would produce similar bands of sensitiveness is to be expected from 
the third conclusion (Draper’s Law) while the reverse would also be 
expected. It is also well established experimentally by the work of 
Von Hubl,* Monpillard,!? and Valenta.**. | 

7 Beitrage zur Photochemie, vol. III, p. 75. 

8 Brit. J. Phot., 1907. 

® Beitrage sur Photochemie, p. 35. 

10 This may be explained by Kundt’s Law or Wiedemann’s theory. See “ Re- 
cent Work in Color Sensitizing,” Wall, Brit. J. Phot., 1907, 51, 406-407. 

11 Brit. J. Almanac, 1906, p. 771, and 1907, p. 744. 

12 Bull. Soc. Photo. Franc., 1906, p. 132. 

18 Beitrage zur Photochemie, III, pp. 153 and 163. 


ee ee ee Oe ee ne 


tl all TT ae a ee ote p i.e, 


ORTHOCHROMATICS ‘Ware 


Theories of color sensitizing involving the fugitive character of the 
dye have been advanced. If this was the case, one would expect the 
dyes having the least stability to light to be the best sensitizers. Ex- 
amination does not show this to be the case. [or instance, cyanin is 
very unstable while erythrosin is quite stable, yet of the two the latter 
is by far the most powerful sensitizer. Also, dicyanine is extremely 
unstable and a weak sensitizer, while rose Bengal is fairly stable and 
yet a good sensitizer. Evidently then there is no connection between 
the fugitive character of a dye and its sensitizing action. 


(MUTT 
rv 


LL 
Bk 


a, 


b= 


Nn l\ al al 


Fic. 129. Drying Cabinet for Sensitized Plates 


It is easily seen how a dye which is fluorescent, if added to the emul- 
sion, might produce color sensitiveness but this theory fails when it is 
shown that some sensitizing dyes, as erythrosine, are not fluorescent. 
Many other fluorescent dyes of similar composition are not sensitizers. 

There is no apparent connection between chemical composition and 
sensitizing properties. The number of useful dyes is small. Good 
sensitizers are found in almost all classes of dyes, while dyes differing 
greatly in stability to light and chemical constitution often show re- 
markable similarity as sensitizers. While there must be some connec- 
tion between sensitizing properties and chemical composition such a 
connection has yet to be discovered. | 

Color Sensitizing.—Plates sensitive to the green and yellow in ad- 
dition to blue and violet are known as orthochromatic or isochromatic. 


178 PHOTOGRAPH. 


Plates sensitive to all the colors of the spectrum are known as pan- 
chromatic. 

In dye sensitizing, absolute cleanliness is essential, as dust or chemi- 
cal contaminations of any kind will cause spots and streaks. Glass 
trays are to be preferred, since they are more easily kept clean than 
trays of other material. Fog in dyed plates may be due to the use of 
the wrong plate, stray light during bathing, use of dye solutions at too 
high temperature, or too much ammonia. For sensitizing to the green 
and yellow it is safe to work by a deep ruby light but for panchromatic 
sensitizing no red light is safe and the operation should be conducted 
in total darkness. | 

Slow drying gives rise to uneven color sensitiveness. The drying 
cabinet shown in Fig. 129 is almost a necessity. The air is driven 
through the light-proof passages by means of the electric fan and the 
constant circulation of air enables the plates to be dried evenly and 


rapidly. Two doors (not shown in drawing) form the front of the 


cabinet and opening them gives access to the interior. ‘The dyed plates 
are placed in ordinary drying racks and distributed along the shelves. 
Sensitizing for Green and Yellow.—The dyes most generally used 
for this purpose are eosin, erythrosine, orthochrom T, pinaverdol, 
pinachrome, and homocol. 
Eosin (C,H,(COC,HBr,OK),O) is soluble in water, alcohol and 
ether. It was advised as a sensitizer by Waterhouse in 1875 but has 


elas dull 
£osin 
Fic. 130. Spectrograph of Eosin. Bureau of Standards paper No. 422 


been completely displaced by the later dyes which confer greater and 
more even sensitiveness. Reference to Fig. 130 will show that its 
sensitizing action extends to about 575 with a decided drop in the blue- 
green at about 540. | 

Erythrosin (C,H,(COC,HI,ONa),O).—Also known as iodeosin 
and bluish eosin. It is soluble in both alcohol and water and a strong 
sensitizer. The curve of sensitiveness (Fig. 131) extends as far as 
580 with a maximum at approximately 540-550. There is a decided 
depression at about 520 in the blue-green. 


ee 


DP ae ee eee Pe 


ORTHOCHROMATICS 179 


In use, one part of the dye is dissolved in one thousand parts of dis- 
tilled water and this stock solution diluted as follows for the bathing 
solution : 


Erythrosin 


Fic. 131. Spectrograph of Erythrosine. Bureau of Standards paper No. 422 


Deer EERO (1 20D0) oi ee. ck bic d eae bes ecvsbeceee 100 parts 
re io ea beaded acta veasaueade 400 parts 
Be reer Ge eee Ne en ey Sewanee dee 5 parts 


(This is a 1: 5000 solution.) 


Rose Bengal (Potassium-tetra-iodo-chloro-flourescein) is also one 
of the eosin group and was formerly used for sensitizing. It is soluble 


Rose Benaal 
Fic. 132. Spectrograph of Rose Bengal. Bureau of Standards paper No. 422 


in both water and alcohol. The band of sensitiveness extends (see 
Fig. 132) to about 600 but shows the depression of sensitiveness to 
blue-green characteristic of eosine at about 400. 


Orthochrome T 
Fic. 133. Spectrograph of Orthochrome T. Bureau of Standards paper No. 422 


Orthochrom T.—This is one of the isocyanine dyes prepared by 
Konig in 1903-4. It is p-toluchinaldin-p-toluchinalinethyl-cyanine- 
bromide and is also soluble in both alcohol and water and sensitizes to 


180 PHOTOGRAPHY 


the blue-green, green, and yellow but has practically no sensitiveness to 
red. (See Fig. 133.) The depression in the blue-green is less notice- 
able than with dyes of the eosin group. 

In use, a stock solution is made containing one part of the dye to 
each 1000 parts of water and this diluted as follows to form the bath- 
ing solution: 

Stock dye solution... :....c.cveecacecsucscees 3 50 2 parts 


Water ......usesw ers cob ne ipipaine so) cllee ew gle Qantas 100 parts 
(This is a 1: 50,000 solution.) 


Time of bathing, three minutes. 
Pinaverdol (Dimethyl-6-methylisocyanine Iodide).—lIts band of 
sensitiveness extends to about 630 which is just a little farther than 


Pinaverdol 
Fic. 134. Spectrograph of Pinaverdol. Bureau of Standards paper No. 422 


orthochrom T. The sensitiveness to blue-green is slightly greater also 
_ than the former dye. It is used in just the same manner as orthochrom 
T (Fig. 134). 

Pinachrome (Diethyl-6-ethoxy-6-methoxyisocyanme Bromide) (1- 
C,H,) (6-OCH,)NC,H, : CH:C,H,N(1—C,H,) (6-OC,H, ) Br.— 


i bea hietniattens fealiivise U 


Pinachrome 
Fic. 135. Spectrograph of Pinachrome. Bureau of Standards paper No. 422 


Soluble in alcohol and water. The band.of sensitiveness extends to 
640, showing a greater sensitiveness to yellow between wave-lengths 
525-625. (See Fig. 135.) Used in the same manner as pinaverdol 
and orthochrom T. 

Pinachrome Blue—vThis is a dye of secret composition introduced 


a a 


ORTHOCHROMATICS 181 


by Konig in 1917. According to Dr. Eder it is an excellent sensitizer 
for dark red to orange and as far as yellow-green. Its sensitizing 
cutve is shown in Fig. 136. 


DOr-™- OW} WM LO w+ + 


Fic. 136. Spectrograph of Pinachrome Blue 


Pinachrome Violet—This dye resembles pinacyanol very closely in 
its sensitizing action. It is a strong red sensitizer through orange and 
yellow to green. There is a small minimum near line C 4% D anda 


SOOG oO ©: © = Con) oO 
NOWMOW O Wo oO Lo oO 
cocmrnm- co oO iO LO + - st 


Fic. 137. Spectrograph of Pinachrome Violet 


pronounced minimum in the green. In comparison with pinachrome 
blue the action does not extend so far into the red but the curve is 
more even. The use of ammonia increases the speed from 4-6 times 
but tends to produce heavy fog (Fig. 137). 


Homocol 


Fic. 138. Spectrograph of Homocol. Bureau of Standards paper No. 422 


Homocol.—This dye sensitizes to about 650 (see Fig. 138) in the 
orange and shows greater sensitiveness between 500 and 600 than 
either pinachrome or pinaverdol, the former more nearly equalling it 
within these limits. It is used at the same concentration and as the 
other isocyanines above. | 

Pinaflavol—This is a green sensitizing dye derived from quinoline 
and prepared by Dr. R. Schuloff of the Hoechst dye works. In com- 
parison with eosin dyes it does not show the deficiency in the blue- 


182 PHOTOGRAPHY 


green which is characteristic of the former but gives a comparatively 
even band over yellow-green and blue. ‘The sensitizing band extends 
approximately from D to F, having a maximum (Fig. 139) at about 


SRT: 


Fic. 139. Spectrograph of Pinaflavol 


530. The drop of the curve at D is of interest, since it means that in 
practice green is represented as brighter than yellow. This, however, 
is incorrect rendering since yellow has the highest visual luminosity 
of any color. For this reason, the dye will probably find its widest 
application in combination with other dyes. The sensitizing bath is as 
follows: 


Pinaflavol stock solution (1% 1000) ..<../<);.:9s ae een I part 
Distilled water. 6.65 i6 0. 62 dss eae chee ole pene ee Ran ee 50 parts 


Time of bathing, 3 minutes. The addition of alcohol at any time 
lowers the sensitiveness according to Konig. 
2-p-Dimethyaminostyrylpyridine Methiodide—This substance was 


Cyanin 


Fic. 140. Spectrograph of Cyanine. Bureau of Standards paper No. 422 


prepared by Mills and Pope in 1922 and is stated to be the most 
powerful sensitizer for green that is known at the present time. Plates 
bathed in an aqueous solution 1 : 30,000 show an almost uniform sensi- 
tiveness from blue to 560, falling off from there to about 620, where 
the action practically ceases. 

Sensitizers for Red.—Cyanine (C,,H,,N.1) (Diamyleyanine-cya- 
nine Iodide).—Also known as chinoline blue. Only red sensitizer 


—s as _——s 


ORTHOCHROMATICS 183 


known for years but very prone to cause excessive fog and now 
superseded by the isocyanines. Its band of sensitiveness (see Fig. 
140) extends to 650. 

Alizarine Blue.—Investigators disagree as to the value of this dye. 
Scoble ** states that he has had good success with the following 
formula: 


ee ET eas rae ge ay alk iowa e bw od ba a0 T's I part 
ee eee eater a. ea, SR re ye ink vein neces sneees 500 parts 
Oe 8 SIRES MORRIS ie 2 a > 10: parts 
ae ARR Ree, Sn SRS Bale, sold sia vie Pe A eee Gn wee ree cs 500 parts 


Filter and use at once before the solution turns blue. The green color 
appears to be necessary in order for the dye to sensitize. The band of 
sensitiveness extends to 900 and it is, therefore, a useful sensitizer for 
work in the infra red. Owing to the deep staining of the gelatine, it 
is not so suitable for general work. 


5c°o0 


ahaiit ee; 


Dicya nin 


Fic. 141. Spectrograph of Dicyanine. Bureau of Standards paper No. 422 


Dicyanine.—This is a strong sensitizer for the infra-red, the sensi- 
tiveness extending as far as 1000 when ammonia is used (Fig. 141). 
Fig. 142 from Walters and Davis ** shows the effect of the addition of 
ammonia. The amount of ammonia which may be used is limited 
owing to the strong fog produced. More care is necessary in the use 
of dicyanine than most of the other dyes and since plates sensitized 
with it do not keep, it is seldom used except in spectroscopic work in 
the infra-red.17 The following is the formula advised by Walters and 
Leavis: =. 

14 Phot. J., 1906, 46, 190. 

15 Bulletin of the Bureau of Standards, 1922, No. 422. 

16 Bulletin of the Bureau of Standards, 1922, No. 422. 

17 See Mees and Wratten, Phot. J., 1908, 48, 25. Dicyanine is now being re- 
placed even for spectroscopic work in the infra-red by neocyanine, a dye de- 


veloped by the Eastman Research Laboratory having a sensitizing band from 
700 to 900. This is the best infra-red sensitizer now available. 


184 PHOTOGRAPHY 


Water cece ce cect ecceceeceeteectcoevet eens bess: ) aq sitet iii 
Ethyl alcohol (os per cent)... i... 20.6 0+00 sus olds cee ann ne 
Dicyanine stock solution (1: 1000) )..+.4..+..s ess eee eee eee 
Ammonia (28 per cent) (C..P oo cscs cisiee «wie cinp con oe iene te 


Dicyanin, water, 
and alcohol 


: 
= 
t 
i 
f 
4 
a 
i 
3 
s 
f 
£ 


Dicyanin, water, alcohol, 
and % per cent am+ 
monia 


Dicyanin, water, alcohol, 
and I per cent ammonia 


Dicyanin, water, alcohol, 
and 4 per cent ammonia 


Fic. 142. Spectrograms showing the difference in the sensitizing action of . 
dicyanin when used in the baths of various composition 

The use of sufficient ammonia in the bath not only increases the sensitizing 
action of dicyanin at its maxima, but extends its action into the infra-red as 
far as 1000 uu 


Dicyanine A.—This is a greenish-blue dye prepared by Dr. E. Konig 
of the Hoechst factory. Its composition has not been published but 


ee ee 


CS ee |. 


Pe ee ne es 


ORTHOCHROMATICS 185 


according to Dr. Eder it is an ethoxy derivative of dicyanine. Ac- 
cording to the same authority its action extends further into the red 
and infra-red than dicyanine, to 780 to 630 in the dark red and weakly 


(SS ae Lae (mf Jad ae RY ao) cs) rs) S 
HOW O LO oS a) LO © 
coc#rnr-cOo CO WO (We) + is 


Fic. 143. Spectrograph of Dicyanine A 


up to 850 in the infra-red. Its minimum from orange to yellow and 
green is more marked, however, than with dicyanine. For very weak 
spectra it is used with ammonia. The plates so prepared do not keep 


well (Fig. 143). 


Pinacyanol*® (1:I'diethylcarbocyanine Iodide) C,H,N(C,H;) : 
CH-CH : CH: C,H,N(C,H,)I.—A plate bathed in an aqueous solu- 
tion shows a band of sensitiveness extending to about 700 in the ex- 
treme red. Ammonia increases the red sensitiveness in proportion to 
amount used. (See Fig. 144.) The use of ethyl alcohol and omit- 


Pinacyanol 


Fic. 144. Pinacyanol. Bureau of Standards paper No. 422 


ting the washing following bathing result in distinctly greater sensi- 
tiveness from 230 to 650. According to Wallace,!® rinsing in alcohol 
after bathing still further increases the sensitiveness. Best concen- 
tration about I : 70,000. 

Naphthacyanole (1: I’diethyl Di-B- ML Mn ovineetar aa. Nitrate ).— 
This is one of the quinoline dyes discovered by the Eastman Labora- 
tory.2° It is a strong red sensitizer showing a strong maximum at 
about 690. (See Fig. 145.) It is thus superior to pinacyanol as a 
red sensitizer. The green sensitiveness, however, is distinctly less than 

18 See Mills and Pope, Phot. J., 1920, 60, 254. 

19 Brit. Journ. Phot., 1908, p. 10%. 


20 Brit. J. Phot., 1922, 69, 474. Also Journal American Chemical Society, 
1920, p. 2661. 


186 PHOTOGRAPHY 


pinacyanol. ‘This dye in combination with other green sensitizing dyes 
would appear to offer great possibilities but at the time of writing 
nothing has been published along this line. 


ys. ai. 
ATM, anit hI 


Fic. 145. Naphthacyanole 


Kryptocyanine.—This is another of the dyes discovered by the East- 
man Research Laboratory and is also a red sensitizing dye. The band 
of sensitiveness extends from 680 to 850 (see Fig. 146) with the 
maximum at about 770. Like naphthacyanole it has little sensitiveness 


P| 
| | | 


ah 
ARAN ARRE,, Piciacaulad 


Fic. 146. Kryptocyanine 


to green. It is expected to be of great service in astronomical work 
with the spectroscope. It is superior to dicyanine for work in the 
infra-red up to 850, beyond which point dicyanine is still unequalled. 
For bathing, a concentration of 1: 500,000 is recommended. The ad- 
dition of alcohol or ammonia is not to be recommended. | 

Pantochrome.—Obtained by the condensation of iodo-ethylate of 
dimethyl-aminoquinaldine with dimethylamine-benzaldehyde. It sensi- 
tizes in a remarkably even band for practically the entire spectrum, 
showing a small minimum at about 500.7? 

Red Sensitiveness with Bisulphite-—Capstaff and Bullock of the 
Eastman Research Laboratory found that bathing an emulsion in a 
two per cent solution of sodium bisulphite for ten minutes at 65 degrees 
Fahr. and washing for times, ranging from five minutes to thirty hours, 
produces remarkable red sensitiveness, which depends on the time of 


21 Bull..Soc. Franc. Phot., 1920, p. 182. 


ORTHOCHROMATICS 187 


washing.” The maximum sensitiveness is produced by soaking for 
ten minutes in a five per cent solution of sodium bisulphite, washing 
for five minutes and soaking for ten minutes in a 0.2 per cent solution 
of pure potassium bicarbonate and finally washing for five minutes. 
Fig. 147 shows the curve of sensitiveness. _ 


2 008 Oe ino ini ek et an... 


deRaE ERASER AERA 


Fic. 147. Red: Sensitiveness with Bisulphite 


Treatment of an ordinary emulsion with iodide confers red sensi- 
tiveness on ordinary plates and films.2* Sheppard finds that the re- 
sults vary considerably with the emulsion, some emulsions being 
strongly red sensitized while others are apparently unaffected.** 

Mixtures of Dyes as Sensitizers.—Pinacyanol and Homocol.—tThis 
combination has been recommended by Monpillard.*®° Without am- 
monia the gain in the red and green is slight but with increasing 
amounts of ammonia the sensitiveness increases. The curve follows 
a fairly straight line with maxima at 580 and 640 to 680 where the 
sensitiveness ends. The formula advised by Wallace ** is as follows: 


ee ere eT OCUS EPS o Soc js. dicy. d ates th ais%as ¥-'e lee bye aie ws ae ee 3 parts 
ee ee ee, eS hdc wg v shiv sle wade wae serine eden 3 parts 
Ir eS te eke eins cl dis Soe teh ve Oe dds wee e's 75 parts 
ONES SS OUD TE A oe a ee 5 parts 
a ee aio k giao nin a oem ey dea eee 5 Mapas 100 parts 


Pinacyanol and Pinaverdol.—These two dyes in combination produce 
a very good color-sensitive plate in which the gap in the blue-green 


characteristic of pinacyanol is greatly benefited. 
Pinachrome and Pinacyanol.*7—The curve of this combination is 
shown in Fig. 148. The first maximum at about 560 is that of pina- 


22 Brit. J. Phot., 1920, 67, 710. 

23 Renwick, Phot. J., 1921, 61, 12. 

24 Phot. J., 1922, 62, 88. 

25 Bull. Soc. Franc. Phot., 1906, p. 132. 


26 Brit. J. Phot., 1908, 55, 102. 
27 From Daur, “ Mixtures of Dyes as Sensitizers,” Brit. J. Phot., 1900, 56, 


504. The three curves show the effect of three different times of exposure. 


188 PHOTOGRAF A 


chrome shifted about 10 wave-lengths to the red. The second at 590 
runs into the other and is probably the pinachrome maximum intensi- 
fied by the pinacyanol. The color sensitiveness is a little less than the 


Fic. 148. Pinachrome and Pinaverdol 


blue sensitiveness and the band of sensitiveness the same as that of 
pinacyanol. 

Pinacyanol-Pinaverdol-H omocol.2®—According to Wallace this is 
an ideal combination, leaving little to be desired in the way of color 
sensitiveness and freedom from fog. When made up in an ammoni- 
acal solution, it sensitizes for practically the whole visible spectrum to 
720. The usual gap in the blue-green is well closed and the curve is 
fairly smooth throughout. The following formula is advised: 


Pinacyanol, 1% 1000. ¢. 0... sce % 01+ 9 oe sd eer ee er 2.5 parts 
Pinaverdol, 1.3 TO00. ... 654/00: sic + «+ nny olsen 2.0 parts | 
Homocol, 1: 1000... feuds ence 2 ay 00s op cyte eee 2.0 parts 
AmMoMia 2.00. ccc cece kd eek Helps de ts ek ee seen 6.0 parts 
Alcohol 2...'.95 sss «oat a oe Pee ee 1.5 parts 
Water: oii icn cdi bance bee we arto Se ae he 100.0 parts 


Time of bathing, 4 minutes. Afterwards immerse in alcohol bath for 
30 seconds. 3 

The Theory of Light Filters—With our present knowledge of 
emulsion making it is impossible to make a plate having the same 
sensitiveness to colored light as the eye. No matter what dye, or 
combination of dyes, is used the action of the blue and violet remains 


28 Brit. J. Phot., 1908, 55, 119. 


Ce ee ne 


£6) Gat tip 


ORTHOCHROMATICS 189 


stronger than it should be. All emulsions are also extremely sensitive 
to ultra-violet, while this is invisible to the eye. To eliminate the 
action of the ultra-violet and diminish the action of the violet and 
blue so as to secure a greater approximation to the sensitiveness of 
the eye, it is necessary to use colored screens which, by absorbing 
these colors either completely or partially, aid the less refrangible 
rays in affecting the plate in approximately the same proportion as 
they do the eye. An orthochromatic filter should, so far as possible, 
completely absorb the ultra-violet without absorbing any of the vis- 
ible spectrum completely, but it must absorb the blue and violet to 
such an extent that the photographic effect of the plate will be equal 
to the visual effect of those colors. Filters which accomplish these 
purposes are known as orthochromatic, compensation, or correction 
filters. ) ! 

While in most cases we desire faithful color rendering, there are 
times when accurate color rendering will not produce a satisfactory 
result and it is necessary to deliberately sacrifice truthful color ren- 
dering in order to bring out the colors satisfactorily. This is due to 
the fact that there are two kinds of contrast by which objects are 
picked out from their surroundings by the eye. We may have color 
contrast where the difference lies purely in color or we may have tonal 
difference where the color is the same in both cases but the two areas 
are different in depth. In the latter case, any plate will properly re- 
produce the contrast provided it is properly exposed and developed. 
In the first case, if the two colors, say green and red, are photo- 
graphed on an ordinary plate, which is insensitive to these colors, both 
are represented by black and consequently there is no contrast. Ifa 
panchromatic plate which is sensitive to the entire visible spectrum is 
used with the proper compensation filter, we secure a uniform field of 
gray without any contrast because of the fact that the two areas are 
different only in color and not in depth or darkness. Therefore, in 
order to bring out the contrast between the two colors, it will be neces- 
sary to sacrifice the correct rendering of either the green or red. 

lf a filter which transmits nothing but green light is placed in front 
of the lens during the exposure, the green will be reproduced light 
while the red will be absorbed in passing through the filter and will 
reproduce dark. If, instead of the green filter, one passing a narrow 
band in the orange-red is substituted, the red will be reproduced as 
light while the green is dark because the green rays from the object 


14 


190 PHOTOGRAPHY 


are absorbed in the filter and fail to reach the plate. Filters which 
show a narrow band of transmission and are used to pick out colors 
from their surroundings are known as contrast or selection filters. 
Orthochromatic Filters—From what has been said before it must 
be evident that the filter must be adjusted to the plate so that the 
proper amount of blue and violet is cut out to make the photographic 
effect approximate the visual effect of color. To find the absorption 
curve of a filter which will give correct color rendering on a particular 


plate we require to know first the sensitiveness of the plate to the 


various colors of the spectrum. If we photograph the spectrum on 
the plate in question and express the densities, which are a measure 
of the work accomplished, as a function of the wave-length, we obtain 
a curve showing the sensitiveness of the plate to the colors of the 
spectrum. Then if from the ordinates of this curve we subtract the 


ordinates of the curve representing the visual intensity of the corre- 


sponding colors of the spectrum we obtain a curve showing the ab- 
sorption which the filter must possess in order to secure correct color 
rendering on that particular emulsion. 3 

In practice, it is not always possible to use a fully correcting filter, 
since the time of exposure may be increased to such an extent, owing 
to the absorption of the active blue and violet rays by the filter, that 
movement will occur. In such cases it is better to be content with 
partial color correction. As an example: three orthochromatic filters 
are supplied by Eastman for the Wratten panchromatic plates and 
panchromatic film. These are designated as K1, K2 and K3 and re- 
quire an increase in exposure of 114, 3 and 4 times respectively. 
These multiplying factors denote the number of times the normal ex- 
posure without a filter must be increased when the filter is used and 
depend on the filter and the plate on which it is used. For instance, 
the multiplying factors of the K1, K2 and K3 screens on a Wratten 
panchromatic plate are 112, 3 and 4% but on the Seed L Ortho plate 
the factors for the same filters are 3,15 and 25. Thus the multiplying 


factor of a given filter is always higher with an orthochromatic plate 


sensitive to green and yellow than with a panchromatic plate which 


is sensitive to the entire spectrum. Of two plates having the same 


speed without a filter, in practice, the panchromatic is faster because a 
lighter screen is required to correct it while at the same time better 
color correction is secured because the latter is sensitive to red while 
the former is not. Most plate makers either make filters for their 


4 
| 


ORTHOCHROMATICS 191 


own products or specify the multiplying factor to be used when an- 
other filter is used. 

Orthochromatic Methods in Landscape Photography.— There is by 
no means complete agreement concerning the value of orthochromatic 
methods among landscape workers. Some workers pin their faith to 
an ordinary plate owing to the better representation of atmosphere. 
Others use color-sensitive plates of the iso type with just enough cor- 
rection to render the clouds with the landscape while still others in- 
sist on complete color correction and use panchromatic plates with 
fully correcting filters. : 

The best methods in practice depend upon the results desired. The 
pictorialist who revels in atmospheric effects of early morn or late 
afternoon and evening will find the ordinary non-color-sensitive plate 
better adapted to his requirements than color-sensitive plates because 
the very deficiency of the plate causes it to emphasize the features 
which he desires. The appearance of atmosphere is due to the light 
rays reflected from dust particles in the air and these rays are always 
either blue or violet, except at sunset or sunrise when they may be 
tinged with yellow and orange. Ordinary plates are very sensitive 
to the blue and violet and also the invisible ultra-violet, which is 
present in the atmosphere to a considerable extent, and, therefore, 
emphasize any suggestion of atmosphere. 

Many workers, other than these, employ orthochromatic methods 
only to the extent of securing printable clouds in their landscapes. 
For this purpose a comparatively light screen is all that is necessary, 
for example a Ki on a panchromatic or a 3 or 4 times filter on an 
orthochromatic plate. When a filter is used in this way, full exposure 
should be given, otherwise the sky portion of the negative is thin and 
the foreground has excessive contrast, sometimes appearing as if 
snow was present. The depth of filter will be determined very largely 
by the strength of the clouds. If these are strongly marked a very 
light filter is all that is necessary, while stronger filters are necessary 
for the thin delicate clouds often observed. Care should be taken not 
to over-correct the clouds (which will be done if a strong filter is used), 
as they lose much of their delicacy and charm when this is done. 

Graduated filters are of great value in photographing clouds in con- 
junction with the landscape. These are filters whose multiplying 
power or depth increases uniformly from o to 10 or 15 times. The 
more actinic blue and violet rays from the sky are made to pass 


192 PHOTOGRAPHY 


through a deeper portion of the filter than the foreground (see Fig. 
149) so that greater correction may be obtained in the sky and the 
extreme distance than in the foreground. By this means, the ap- 
pearance of over-correction in the foreground is avoided while the 
distance and clouds are sufficiently corrected to enable them to print 
with the proper contrast. : 

For panoramic work of mountain scenery and great distances, com- 
paratively strong filters are necessary in order to eliminate the haze 
in the atmosphere. Sometimes on very clear days a K3 filter may be 


Fic. 149. Action of Graduated Filters 


sufficient but in most cases stronger filters will be required, such as 
the G “strong yellow ” requiring an increase in exposure of 6 times 
with panchromatic plates and 30 times with the Seed L Ortho plate. 
In extreme cases,.even deeper screens will be necessary as for ex- 
ample the “ B ” screen and the Orange Red “ A ” which require an in- 
crease of 10 and 12 times with panchromatic materials. The multiply- 
ing factor of the former on a Seed L Ortho plate is 12 while the latter 
is not suitable for use with plates of this type since they are insensitive 
to the light which it transmits. 

Orthochromatic Methods in Portraiture-—Concerning the value of 
color-correct rendering in portraiture, Dr. Mees says:7® “In no 
branch of photography is the reproduction of colored objects in mono- 
chrome of greater importance than in portraiture and in no branch 
is it in greater danger of being ignored. The flesh tints, with which 
the portrait photographer is mainly concerned, are chiefly of a red- 
dish nature, while the yellow and brown shades of hair and the va- 
riety of eye-colors apart altogether from the clothing cause every sit- 
ter to present a distinct problem in color reproduction.” 

Figure 150 is a print from a negative made on an ordinary non- 


29 Photography of Colored Objects, 


193 


ORTHOCHROMATICS 


SUOISINWIA Ij}eWOIyIUegG pue AJeUIpPIQ UO eI}10g 


oSI 


3) a | 


194 PHOTOGRAPHY 


color-sensitive portrait plate. Ordinary plates are sensitive only t 
the violet.and to blue, rays which are almost completely absorbed by 
the skin. The result is that an ordinary plate fails to reproduce the 
~ texture of the skin properly and produces excessive contrast which 
emphasizes all of its lines and imperfections. The various shades of 
brown, golden and red hair are difficult to photograph properly and 
all sorts of dodges are used by operators to secure a passable render- 
ing of the same. In most cases when the proper tint is secured the de- 
tail of the shadows in the hair is lost. The wrinkle which exists 
around the eyes is often a comparatively deep shade of red and is 
reproduced too dark with an ordinary plate and the retoucher in 
lightening the same often destroys the strength of the eye by taking 
out the wrinkle entirely. | 

In Fig. 150b is shown a portrait of the same subject under identical 
- conditions excepting that a color-sensitive plate and filter were used 
in making the original negative. The material used was the East- 
man Panchromatic film and a Ka filter requiring an increase in ex- 
posure of 3% times. The marked improvement in the rendering of 
flesh tones and skin texture is quite evident. While the result may 
not yet be entirely satisfactory and some further retouching may be 
necessary, considerably less time will be required for this operation 
since most of the retoucher’s work has been done for him, and owing 
to the comparatively small amount of retouching required on the lat- 
ter negative there is less danger of losing the facial expression of the 
subject in that operation. 

It is difficult, on face of the above facts, to see why color-sensitive 
plates are not used more widely in portrait photography. Formerly, 
the greatest objection to their use was that in order to secure any 
advantage over the ordinary plate a filter was necessary and this 
increases the exposure several times so that there is greater danger 
of movement. Now, both orthochromatic and panchromatic plates of 
high speed are readily obtainable and with the general adoption of 
artificial light there is no real reason why color-sensitive products and 
filters cannot be used with complete success. For all ordinary work 
the Ki filter requiring an increase of 50 per cent in exposure will give 
sufficient correction on panchromatic plates while with certain sub- 
jects a K2 filter which requires about three times more exposure will 
be necessary. As panchromatic plates of high speed are now avail- 
able, these exposures should not be unduly long and the advantages 


ORTHOCHROMATICS 195 


secured by the use of color-sensitive products far outweighs the oc- 
casional loss due to movement of the subject. 

Most artificial lights are deficient in blue rays and have greater 
intensity in the red than daylight. Fig. 151 shows the distribution of 
energy in the spectrum of daylight and gas-filled (tungsten) electric 


490 soo 699 
4 Pahtria Gionibidlats voller aniPleree Voen 


Sunlight 


Tungsten lamp 


Fic. 151. Spectrum of Daylight and Mazda Clear Glass Bulbs 


lamps. Under these conditions color-sensitive plates are distinctly 
faster than ordinary plates owing to their greater sensitivity to the 
green and red. Furthermore, owing to the spectral distribution of 
light from artificial sources of this type, filters are not needed except 
where extra correction is necessary. For all ordinary portrait work 
with gas-filled tungsten lamps a panchromatic plate without a filter 
will give sufficient correction and is faster than an ordinary non-color 
sensitive plate having the same speed in daylight. 

Photographing Color Contrasts.—We referred to this subject under 
the subject of color filters but we now wish to devote some space to 
the application of the same in practice. 

To photograph a color as black a filter must be employed having an 
absorption band in the wave-lengths of the particular color to be 
rendered as black. In other words to photograph any given color as 
black it must be photographed through a sharp cutting filter which 
completely absorbs the color of the subject. No rays of light re- 
flected from the subject will then reach the plate and the color will be 
as black as it can be made. 


196 PHOTOGRAPHY 


ney 

Rt most valuable publication’ of the year is the 
"mistorie de la Decouverte de la Photographie by Georges Potonnice 
[published by Montel-Paris). In this work the history of photo- 
graphy is' covered completely from its inception to the death of 
Daguerre in 1851. The second volume (to appear soon |will complete 
the work and bring it down to modern times. Another notabte work 
of the year is the Physics of the Developed Photographic Tuace 
by f.E.Ross,which is Number 4 of the series of Yonographs on the 
jae ot DunORFaiiE issued by the Research Levoratory of the 
Eastman Kodak Company. Many of the more important sensitizing 
and desensitizing dyes are afsnueaed in a work by JeF.Rewitt 
"Dyestuffs « derived from | Pyridine, quinoline ,Acridine (Snd 3 Xanthene". 
(Longemanbs Green & Co., New York) 


Fic. 152a. Photograph of Manuscript in Blue with Red Corrections using 
Green Filter 


Perhaps the most valuable publication of the yet the 
Historie de la Decouverte de la Photographie by Georges Potonniee , 
[published by dontercbenteie In this work the history of photo- | 
graphy is covered completely from its inception to the death of 
Daguerre in 1851. The second volume to appear soon will complete 
the work and bring it down to modern times. Another notabie work 
of the year is the Physics of the Developed Photographic image 
by f.E.Ross,which is Number 4 of the series of monographs on the 
theory of photography issued by the Research laboratory of tie 
Eastman Kodak Company. Many of the more important sensitixing 
and desensitizing dyes are discussed in a work by J.F.Mewit 
‘Dyestuffs derived from Pyridine,Quinoline,Acridine and Xanthene". 


(Longsmanns Green & Co., New York) 


Fic. 152b. Photograph of Manuscript in Blue with Red Corrections 
showing Use of Red Filter 


ORTHOCHROMATICS 197 


-To render a color as white it must be photographed not in its ab- 
sorption band but in its reflection band, ..In other words, any color 
will be reproduced as light if it is photographed through a filter of its 
own color. 


Red objects absorb blue and green light. 

Green objects absorb blue and red light. 

Dark Blue objects absorb green and red light. 
Yellow objects absorb blue light. 

Magenta or purple objects absorb green light. 
Light blue or blue-green objects absorb red light. 


Suppose, for instance, we have a manuscript typewritten in blue ink 
with corrections in bright red. We desire to make one photograph 
showing the manuscript complete with corrections and another show- 
ing the text without the alterations. What filters must we employ? 
If an ordinary, non-color sensitive plate without a filter is employed we 
will probably find that while the alterations in red stand out while the 
blue of the original text is quite faint. An orthochromatic plate with 
a compensating filter will make the blue typewriting somewhat darker 
but for the greatest possible contrast we must employ a contrast filter 
which completely absorbs both blue and red. Such a filter would 
transmit a narrow band in the green and would give us the result 
shown in Fig, 152a. To eliminate the corrections we must reproduce 
red as white while making blue dark, accordingly we would select a 
contrast filter transmitting red, such as the Wratten A or F. This 
would give us the result shown in Fig. 152b. Should it be required to 
photograph the corrections alone, eliminating the original blue type- 
written text, this might be accomplished by the use of a dark blue filter, 

such as the Wratten C. 

One of the best examples of the value of orthochromatic methods 
and the application of the principles of color contrast occur in photo- 
_graphing furniture. In Fig. 153 are shown comparative photographs 
of wood sections on ordinary and panchromatic plates with proper 
filters and the immense improvement in results obtained by the use of 
the latter is at once evident. If red mahogany, for instance, is photo- 
eraphed on an ordinary plate, no trace of the grain is visible, while 
increasing the exposure merely results in bringing up a large number 
of scratches imperceptible to the eye. However, by using a panchro- 
matic plate with an orange-red filter the scratches disappear and the 
erain of the wood is brought out. 


198 PHOTOGRAPHY 


RCASSIAN WALNUT | 


CIRCASSIAN 


Fic. 153. Wood Sections on Ordinary and Panchromatic Plates 


FEAST INDIAN WALNUI ff 


INDIAN TEAK 


> 


= — tae 4 = co A ws .> fs a me - al pa 7" oo ae i mike “ 
ee ae ee ee ee eee ee ee ae ee: ee ee ee en. ee ee ee ae 


Ordinary 
Plate 


On 
Panchro- 
matic 
Plate 


(Courtesy of Ilford Ltd.) 


ORTHOCHROMATICS Lug 


In photographing furniture, success depends chiefly upon the selec- 
tion of the proper filter for the subject. For mahogany the greatest 
contrast is obtained by using an orange-red filter such as the Wratten 
A. With yellow woods like oak, satinwood, and walnut, the deep yel- 
low filter as the Wratten G will be of greatest service. Care must be 
taken not to exaggerate the contrast of inlaid furniture and the mat- 
ter must be compromised, using either a fully correcting orthochro- 
matic filter as the K3 or one of deep yellow or orange-red. | 

In general it is best to depart from orthochromatic rendering only 
when absolutely necessary. Whenever there is doubt, it is good policy 
to make one exposure with an orthochromatic filter in addition to that 
made with the contrast filter which is judged to be correct. 


BIBLIOGRAPHY 


GENERAL REFERENCE WorRKS 


BakeR—Orthochromatic or Isochromatic Photography. 

Eper—Uber die Chemischen Wirkungen des Farbigen Lichtes. 

Eper AND VALENTA—Beitrage zur Photochemie und Spectralanalyse. 
Husrt—Die Photographischen Lichtfilter. 

Husit—Die Orthochromatische Photographie. 

Konic—Das Arbeiten mit Farben empfindlichem Platten. 
Meres—Photography of Colored Objects. 

WEIcERT—Die Chemischen Wirkung des Lichts. 


CHAPTER VIII 
THE LATENT IMAGE 


Photo-Physical and Photo-Chemical Change.—Nearly every body 
undergoes some change when exposed to light. The change may be 
slow or it may be remarkably rapid, as in the case of the silver halides, 
according to the nature of the body, and it may be either physical or 
chemical in character. In the first case the change consists in an 
alteration of the appearance or properties of the substance but unac- 
companied by any change in composition, while in the second case the — 
composition, as well as the properties of the substance, are altered. 
As an example of a physical change due to the action of light we may 
take selenium, which in darkness is a non-conductor of electricity but 
becomes a conductor when exposed to light. Yellow phosphorus, a 
highly inflammable substance, is gradually converted by the action of 
light into a red phosphorus with entirely different properties. Pow- 
dered non-crystalline selenium gradually becomes crystalline upon ex- 
posure to light. Certain metallic salts, such as the crystalline chloride 
or iodide of silver, nickel sulphate, and zinc selenate, experience a — 
change in crystalline form under the influence of light. In all such 
cases it should be observed that no chemical change has taken place. 
Crystalline and non-crystalline selenium are both selenium and have 
the same composition, while the same is true of the forms of yellow 
and red phosphorus and of soluble and insoluble sulphur. The change 
which has taken place is due to some alteration in the arrangement of 
the molecules but not to such an extent as to cause a chemical change. 

Regarding the chemical changes due to light, Eder has made the 
following general statements: 

1. All kinds of light from the ultra-violet to the infra-red, whether 
visible or not, have some photo-chemical action. The rate of action 
may vary to a considerable extent, but there is no kind of light that is 
absolutely without effect on a body if the time is sufficiently prolonged. 

2. Photo-chemical action is produced only by such rays as the body 
absorbs (Draper’s Law), so that the chemical action of light is closely 
related to optical absorption. 

3. The sensitiveness of a body towards rays of a definite refrangibil- 
ity is increased by the admixture of other substances which absorb © 
the same rays. | 


ee ee ee Se ee Le a Pe ee OR 


ee a eR Oe ne ee Se ee ee 


200 


THE LATENT IMAGE 201 


4. A substance is, as a rule, decomposed faster by a given color 
when it is mixed with a body which absorbs one of the products re- 
sulting from the photo-chemical decomposition. 

The action of light may bring about either decomposition or com- 
bination. Examples of the former occur in nearly all photographic 
processes while a familiar example of the latter is the union of chlorine 
and hydrogen, to form hydrochloric acid according to the equation: 


H, + Cl, = 2HC. 


Moisture is essential to the above reaction and it is possible that a cer- 
tain amount of water is required for all photo-chemical reactions. 

Thus the action of light may be either reducing or oxidizing in 
character, depending upon the nature of the substance under its in- 
fluence. ¢ 

The Latent Image.—When light is allowed to fall on a photographic 
plate, or upon silver halide precipitated from solution, the silver bro- 
mide is altered in some unknown way because a reducing agent, or 
“developer,” is able to darken the silver bromide exposed to light 
more rapidly than that which has not been exposed. We say that the 
light has produced a “ latent image” because it is invisible to the eye 
but susceptible to certain reducing agents, and it is our problem to de- 
termine the nature of this change and of what this latent image con- 
sists. The nature of the change which occurs when a silver halide is 
exposed to light is still an unsolved problem, despite much speculation 
and the enormous amount of experimental work which has been done 
by the most eminent scientists in an attempt to reach a solution of the 
problem. While this work has not enabled us to reach any definite 
conclusion, it has been of very real value as many facts regarding the 
character and reactions of the invisible image have been established 
which must of necessity be taken into consideration when forming a 
working theory of the latent image. Therefore it seems advisable to 
review some of the more important experimental work by various 
authorities, which has a definite bearing on the nature and composition 
of the latent image, before proceeding to a discussion of the theories 
advanced to explain the same. . 
Artificial Latent Images.—Light is not the only agent to which the 
silver halides are sensitive and several other agencies are known to 
form latent images. According to Namias? a 1: 20,000 solution of 
crystallized stannous chloride (SnCl,°2H,O) in distilled water will 


1 Phot. Korr., 42 (1907), p. 155. Jour. Phys. Chem., 14 (1910), p. 326. 


202 PHOTOGRAPHY 


produce a latent image which may be detected with ordinary developers. 
The action seems to be exactly the same as that produced by light. 
The action is stronger in more concentrated solutions, while if the solu- 
tion is very strong the plate fogs badly and the effect is very similar to 
that of over-exposure. If the action is prolonged beyond this stage, 
a visible image is formed which appears to be analogous to that pro- 
duced by the continued action of light. 

Towards the end of the last century, W. J. Russell found that many 
_ substances were able to act on a photographic plate in some manner so 
as to make it developable without any exposure to light. The num- 
ber of substances which would act in this manner was very great, and 
included freshly scratched metals, especially zinc and magnesium, 
many fats and volatile oils, and numerous other natural organic bodies 
like wood, straw, blood and resin. The activity of all these materials 
was traced to the formation of hydrogen peroxide as a result of the 
superficial oxidation of the substances in moist air. The vapor and 
solution of hydrogen peroxide itself exhibited the phenomenon to a 
much more marked degree. 

Since Russell’s first experiments, a vast number of materials have 
‘been discovered which, when applied to a plate, make it developable 
in absence of exposure to light. For instance solutions of many mild 
reducing agents such as sodium arsenite, very dilute ferrous oxalate, 
sodium hypophosphite and stannous chloride, dilute acids, certain 
neutral salt solutions and some dyes can all act on a plate to give some 
sort of latent image. ‘The materials which have been most investigated 
in this respect are sodium arsenite and hydrogen peroxide. Their ac- 
tions on the plate show an extraordinary parallelism with the action of 
light. A study of the fogging action of peroxide and arsenite should, 
therefore, be of assistance in shedding some light on the nature and 
formation of the latent light image, and on the nature of the sensi- 
tiveness of the grains in a photographic emulsion. 

Hydrogen Peroxide.—The action of hydrogen peroxide increases 
with increase in time of treatment? of a plate by a solution of definite 
concentration, and with increase in concentration of the solution, for 
a given time of treatment, giving rise on development to a density- 
exposure curve similar in form to the well-known S-shaped character- 
istic curve for exposure to light. On prolonged treatment with per- 
oxide the curve shows a definite reversal portion, as in the case of 
light exposure. The characteristic curve for peroxide treatment varies 


2S. E. Sheppard and E. P. Wightman, J. Franklin Inst., 1923, 195, 337. 


THE LATENT IMAGE 203 


in the same way with time of development as does the normal curve 
for exposure to light. Plates most sensitive to light are also most 
sensitive to peroxide, and the bigger grains in an emulsion are, on the 
average, more sensitive than the smaller ones to both light and per- 
oxide.* ‘ 

In view of this extraordinary parallelism it is difficult to believe that 
the actions of peroxide and of light in forming the latent image are not 
ultimately the same. In fact, theories have been proposed to account 
for the action of peroxide as due to the emission of radiation as a re- 
sult of the decomposition of the peroxide.* There is evidence, how- 
ever, which is contrary to this chemiluminescence view, so that it is by 
no means generally accepted. 

Owing to the uncertainty as to the action of peroxide, the study of 
the fogging action of sodium arsenite is of interest, as in this case the 
emission of radiation cannot so reasonably be postulated. An aqueous 
solution of mono-sodium arsenite (NaH,AsO,) reproduces the action 
of light on a plate as faithfully as does hydrogen peroxide, and it has 
been studied in more detail in certain respects.® 

Sodium Arsenite——Sodium arsenite gives a characteristic curve 
similar to that for light, and with a well-defined reversal portion. 
Plates faster to light seem also to be more sensitive to the action of 
arsenite. The distribution of the latent image due to arsenite treat- 
ment has been studied in the same way that Svedberg and Toy studied 
the distribution of the latent light image due to light, by making 
statistical measurements on the “reduction centers” shown up by 
partial development of the emulsion grains. The “ reduction centers ” 
in the silver halide grains in an emulsion can be shown up after treat- 
ment with arsenite in a manner similar to that in the case of light, and 
they are found to be distributed among all the grains, and topographi- 
cally on the individual grains themselves, according to the same laws 
as are found to hold in the case of exposure to light. 

It is possible, by using the p-phenylenediamine-silver sodium sulphite 
mixture, to develop physically, after fixation, the latent image due to 
sodium arsenite. Treatment of a plate with chromic acid solution 
desensitizes it to the action of sodium arsenite in the same way as to 
light action. In the desensitization of a plate to light by chromic acid, 


8 Luppo-Cramer, Phot. Korr., 1902, 643; 1003, 89; 1908, 548. Graetz, Z. 
Physik., 1902, 5, 160; 1903, 9, 271. Svedberg, Z. wiss. Phot., 1920, 20, 37. 

4 Sheppard and Wightman, J. Franklin Inst., 1923, 195, 337. 

5 Bancroft and Perley, J. Phys. Chem., 1910, 14, 292, 648. Clark, Brit. J. 
Phot., 1922, 69, 462; 1923, 70, 717. Clark, Phot. J., 1924, 64, 363. 


204 PHOTOGRAPHY 


it is found that a preliminary exposure to light before bathing in 
chromic acid greatly accelerates the rate of desensitization. That is, 
the latent image is attacked by chromic acid much more readily than 
the sensitive nuclei themselves. The same is found to hold if the 
“preliminary exposure ” is treatment with sodium arsenite solution.® 

The formation and reaction of the latent arsenite image are thus 
very similar to those of the latent light image. 

Now, these observations with sodium arsenite are of vital impor- 
tance, if it can be accepted that there is no interaction between sodium 
arsenite and silver bromide. For then we are faced with the fact that 
silver bromide does not react with sodium arsenite, and yet is made de- 
velopable by it. If we neglect the improbable suggestion that the 
arsenite acts by breaking down the protective action of the gelatin on 
the grains, the only conclusion we can come to from the observations 
is that the latent arsenite image is due to reaction of sodium arsenite 
with something at the grain surface which is not silver bromide. Now, 
it is clear from a study of the action of arsenite that it is extremely 
probable that both sodium arsenite and light act at the same points in 
the grains, so that it appears that light acts at points where there is 
some material other than silver bromide—that is, the sensitiveness of 
grains in an emulsion is due to the presence of traces of material not 
silver bromide, which is distributed in spots haphazard among the 
grains. ; 

As stated, these considerations are only valid if there is no interac- 
tion between sodium arsenite and silver bromide. It has been found 
that in presence of alkali the monosodium arsenite, which under these 
conditions is mixed with the higher sodium salts, can react with silver 
bromide, giving a complex which is unstable and slowly deposits silver 
on standing. With the monosodium arsenite, prepared from arsenious 
oxide and caustic soda, some workers claim to have shown an interac- 
tion of silver bromide, although in the cases studied by Clark no sug- 
gestion of any action could be demonstrated, and yet a very marked 
effect was obtained on the plate.’ 

Reversal by Light.—With a short exposure to light we get a latent 
image which on development yields a negative. If the exposure is 
lengthened considerably the image becomes positive instead of negative 
when developed, while still further exposure will produce a second 
negative and it is probable that the cycle may be repeated indefinitely, 


6 Clark, Phot. J., 1923, 63, 237; 1924, 64, 91. 
* Luppo-Cramer, Phot. Ind., 1923, 456. Clark, Brit. J. Phot., 1923, 70, 717. 


———— eee 
- 
- 


LHe LATENT IMAGE 205 


although owing to the enormous exposures required no one has been 
able to go past the second negative stage, so far as the writer is aware. 

No photographic process is, strictly speaking, free from the effects 
of reversal, but rapid gelatino-bromide plates are more subject to the 
defect than a comparatively insensitive plate, such as wet-collodion. 

The conditions leading to reversal and the peculiarities of the phe- 
nomenon have been studied by many, among whom may be mentioned 
Abney,® Janssen,? Crowther ?° and Preobrajensky."! 

It has been determined that atmospheric oxidation is probably 
necessary, also, that a preliminary exposure to light aids reversal, 
while oxidizing agents also facilitate reversal, but reducing agents 
either prevent it altogether or retard its appearance. The red rays 
were found by Abney to be more active in producing reversal than 
those of shorter length. (See Treatise on Photography, 1oth Ed., 
1918, pp. 93 to 97.) 

Reversal by Chemical Reagents.—The function of exposure of a 
plate is to affect the grains at the points of sensitivity in such a way 
that nuclei are formed which are sufficiently big to act as deposition 
centers for the development process. The function of fogging agents 
such as have been considered must be a similar one. In the reversal 
process with light it is probable that the function of prolonged ex- 
posure is to make the deposition centers inactive again. It is sug- 
gested by some that this could occur by some sort of “ retrogressive ”’ 
action, the centers reverting to their original state; but actually such 
a reversion seems to be thermodynamically impossible as long as 
the light stimulus is acting. The more probable result of prolonged 
exposure is to bring about some “ progressive ” action which so changes 
the centers as to make them no longer able to function as centers for 
development. How this occurs is not clear. In the case of arsenite, 
however, a very satisfactory explanation is found in assuming that on 
prolonged treatment the arsenite peptizes the nuclei formed in the first 
stages of its action, and so makes them too small to function in de- 
velopment. This view is supported by the experimental observation 
that sodium arsenite can peptize colloidal silver in gelatin, and also 


8 Abney, Instruction in Photography, pp. 33-35, 10th Ed., 1901; Treatise on 
Photography, pp. 93-97, 10th Ed., 1918. 

® Janssen, Compt. rend., June, 1880. 

10 Crowther, Phot. J., 1914, 54, 253, and Phot. J., 1915, 55, 186. 

11 Preobrajensky, Bull. Soc. franc. Phot., 1906, pp. 124, 281; Phot. J. (1906), 
46, 371, and (1907) 47, 335. 


15 ' 


“tet am 


206 PHOTOGRAPHY 


that it can destroy the latent image left after fixation of an exposed 
plate, so that it cannot be physically developed.*? 
_ It is seen, then, that although latent image formation is similar in 
the case of arsenite and of light, the reversal process is probably quite 
different in the two cases. The presence of the latent image alone is 
2 sufficient condition for reversal by arsenite, but for reversal by light, 
the silver halide itself must also be present. Hydrogen peroxide solu- 
tion can also peptize colloidal silver and destroy the latent image, so 
that an explanation of reversal by peroxide solution similar to that 
advanced for arsenite is satisfactory. In the case of reversal by ex- 
posure to hydrogen peroxide vapor, however, it is more difficult to con- 
ceive that it is due to peptization.'* 

Although it is difficult to obtain really direct eee concerning the 
way in which many fogging agents act, and results and opinions con- 
cerning their action are somewhat conflicting, enough reliable data have 
been obtained to indicate that the study is of great importance for the 
theory of photographic sensitivity. In fact, it has played an important 
part in leading up to the modern conception of sensitivity as due to the 
presence on the silver bromide grains of traces of some substance not 
‘silver bromide. 

Photo-Regression.—With a daguerreotype plate, development has 
to be done immediately after the exposure as the image cannot be 
retained for more than a few hours and gradually grows weaker after 
exposure. The same condition of affairs applies to the wet collodion 
plate, although here the loss of the image may be ascribed to the 
physical condition of the collodion which requires a certain amount of 
moisture. With gelatine plates the image is remarkably permanent 
and instances are on record where gelatine plates have been success- 
fully developed several years after exposure. 

The gradual disappearance of the image after exposure and before 
development is termed photo-regression, and appears to be a process 
exactly the reverse of that which produces the latent image. Accord- 
ing to Baekeland** photo-regression is more apparent on images 

12 Phot, J., 1924, 64, 363. . .* 

18 Phot. J., 1924, 64, 363. Cf. also Wightman, Trivelli and Sheppard, J. 
Franklin Inst., 1925. 

oll > Wak F Chavaom “ Effect of Time on the Latent Image,” Phot. Ty 1917, 57, 


72, 
15 Zeit. wiss. Eat 1905, 3, 58. | 


a 
a 
j 


TL)? oY 


Pee ee a a ee een 


CE , eee e ee Oe e 


ee al oe 


pe ere ee 


ee ee leet ee ek nee, Coren id 


Me AC RIIN Er WEAN 207 


which have received less than normal exposure, and the developing 
agent used for developing appears to have no effect on the final result. 
The factors which appear to have the greatest influence on the rate at 
which the image disappears are temperature and humidity, while the 
presence of alum or free acid in the emulsion also plays an important 
part. The higher the temperature and the humidity in which plates 
are stored after exposure and before development the more rapid is 
the disappearance of the image. Plates or papers which contain alum, 
or those in which the emulsion is in an acid state, are more subject 
to rapid disappearance of the image than plates which do not contain 
alum, or in which the emulsion is in a neutral or slightly alkaline state. 
The action of alum no doubt explains why developing-out papers are 
more subject to photo-regression than plates, since alum is always 
added to the former in order to render the gelatine less soluble, while 
plates do not need additions of alum unless made for use in very hot 
climates. According to Luppo-Cramer the size of grain has an in- 
fluence, small-grained emulsions showing regression more rapidly than 
those of coarser grain. 

The phenomenon of photo-regression is interesting in that it shows 
that the sensitive plate has a certain faculty of self-recovery from the 
effects of light and any workable theory of the latent image must 
satisfactorily explain the reason for the same, before it can receive 
serious consideration. 

The Action of Solvents of Silver on the Latent Image.—The theory 
has been advanced that the latent image consisted of “germs” of 
metallic silver which were produced by the reduction of the silver 
bromide by light. In combating this theory Dr. Eder contended that 
if the latent image consisted of metallic silver, it should be soluble in 
a silver solvent, as nitric acid, and that this was opposed to the ex- 
perimental evidence which he had obtained. Since the effect of silver 
solvents on the latent image had an important bearing on the metallic 
silver theory, the question was studied rather carefully both by those 
who opposed and by those who favored this theory. Luppo-Cramer 
using collodion emulsion found that 33 per cent nitric acid had little 
or no effect on the latent image if applied before exposure, but if 
applied after the exposure the image was partially destroyed and only 
the highlights remained. Using concentrated 65 per cent nitric acid 
the image was completely destroyed. In supporting the metallic silver 
theory Abegg also came to the conclusion that the latent image was 
destroyed by nitric acid. 


208 PHOTOGRAPH: 


Later Dr. Eder modified his original statement that the latent 
image was not destroyed by nitric acid and came to the following 
conclusions : *° 

1. The normal briefly exposed latent image on collodio-bromide 
plates is completely destroyed by nitric acid. 

2. The longer exposed latent image is not totally destroyed but is 
weakened. | 

3. The solarized latent image is only slightly attacked by the 
strongest nitric acid and develops as a thin negative instead of a 
positive. 

It is therefore now generally admitted that in the case of a short 
exposure at least the latent image is destroyed by nitric acid. 

According to Mercator" other acids which are not solvents of 
silver will also destroy the latent image. 

Physical Development of the Latent Image after Fixation. 4g a 
plate is fixed in hypo directly after exposure one would assume that 
the image would be destroyed, since hypo is a solvent of the silver 
halides. Such, however, is not the case for as shown by Young in . 
1858 with wet collodion and by Kogelmann, Sterry, Neuhauss, 
Lumiere and Seyewetz and others with gelatine emulsion, the latent 
image is not destroyed by fixing but may be developed in a physical  __ 
developer, i.e., a developing solution containing in addition to the 

| 


; . 
: . - 
a ee ae, ee ee ee ee eee le 


developing agent a silver salt capable of forming silver in the nascent 
state.*® 

Lumiere and Seyewetz found that the latent image is partially de- 
stroyed by prolonged fixing in hypo and by acids. By fixing five min- 
utes in a 30 per cent solution of hypo with the addition of one per 
cent of ammonia and washing in water made distinctly alkaline the 
latent image is not reduced and the process can be worked with but 
little more exposure than that required for normal exposures. 

Physical development is supposed to be due to the attraction of 
the nuclei of the latent image for the nascent silver of the developing 
solution. Assuming this explanation to be correct, then any solution 


16 The most convenient reference is Brit. J. Phot., 1905, 52, 950, 968. 

17 Brit. J. Phot., 1899, 46, 628. 

18 Young, Photographic News, 1858, 1, 165. Kogelmann, “ Die Isolierung der 
Substanz der Latenten, Photographischen Bilder.” Graz, 1899. Sterry, Photog- 
raphy, 1898, p. 260. Neuhauss, Phot. Rund., 1899, p. 257, and 1904, p. 54. 
Lumiere and Seyewetz, Bull. Soc. franc. Phot., 1911, pp. 264, 373; 1924, p. 169. 
Compt. Rendus, 1924, 179, 14. Luppo-Cramer, Phot. Rund., 1924, p. 780. 


THE LATENT IMAGE 209 


which will reduce a silver salt to the nascent condition should be able 
to develop the image, although not a developing agent in the com- 
monly accepted sense of the term. 

To confirm this point Lumiére and Seyewetz *° tried as a developer 
a solution of silver sulphite in excess of sodium sulphite and formalde- 
hyde. No image was obtained, however, showing that the nuclei left 
after fixation are incapable of attracting nascent silver. However, if 
the fixed-out plate be first immersed in paraphenylenediamine or 
amidol the nuclei acquire the property of attracting the nascent silver 
and physical development becomes possible. 

From which we may conclude that the nuclei left after fixing can- 
not be silver bromide, for hypo is a solvent of the silver halides, nor 
from the above experiment can they be regarded as metallic silver un- 
less its property of attracting nascent silver has been destroyed by un- 
known factors. 

The Photosalts.—In 1887, a brilliant American chemist, Carey Lea 
of Philadelphia, succeeding in preparing compounds of silver chloride 
which contain less halogen than the original chloride, by treating 
ammoniacal solutions of silver chloride with ferrous sulphate, wash- 
ing the precipitate, and then treating with hydrochloric acid. A large 
number of these compounds were prepared by their discoverer and 
were called “ photosalts,” because he considered them identical with 
the compounds formed when silver chloride is exposed to light. The 
photosalts were considered to be definite chemical compounds by their 
discoverer, but most investigators took the view that the combination 
was more of the character of a “ lake,” or a physical combination of 
the altered and unaltered haloids. A theory that the latent image 
consisted of a solid solution of silver sub-bromide in silver bromide 
was advanced by Lea himself *° and was supported by Luppo-Cramer 
and Lorenz.?? 

Some very valuable experimental work on the preparation and 
composition of the photosalts has been done by a number of German 
photo-chemists in recent years and especially by Reinders and 
Weigert. (See bibliography.) 

Image Transference.—According to Renwick, Eder and Pizzighelli 
in 1881 were the first to show that an exposed silver chloride plate 

19 Phot. Revue, 1925, 37, 48. 

20 Lea, American Jour. Sci. (3), 33, 1887, 349, 480. 


21 Luppo-Cramer, Phot. Korr. (1906), 43, 388, 433. Lorenz, Phot. Korr. 
(1901), 38, 166. 


210 PHOTOGRAPHY 


could be converted by treatment with potassium bromide into silver 
bromide without any loss of the developable condition. In other 
words it is possible to transfer the latent image from one halide to 
another without destroying its capacity for development in those 
parts where the light acted. This phenomenon is known as “ image 
transference.” ?° | 

In his Hurter lecture before the Liverpool Section of the Society 
of Chemical Industry, 1920, Renwick showed that if after exposure 
the plate is immersed in a 1 per cent solution of potassium iodide, 
which contains 2 to 3 per cent of a neutral sulphite, all of the silver 
salts are converted into silver iodide. After washing in a dilute neu- 
tral sulphite solution, the silver iodide image may be developed with 
a ¥%4 per cent solution of amidol and 1o per cent each of crystal car- 
bonate and sulphite of soda. Owing to the insensitiveness of silver 
iodide when precipitated in an excess of a soluble iodide it is possible 
to develop plates after iodizing in strong white light. However, if 
any of the original silver bromide remains it will immediately blacken, 
owing to the superior sensitiveness to light. That such does not hap- 
pen when the plates have been thoroughly iodized may be taken as 
evidence of the fact that the latent image has been transferred from 
one halide (the bromide) to another, which is in this case silver 
iodide. 

Indoxyl Development.—The oxidizing properties of the latent image 
have been used repeatedly as an explanation of the process of de- 
velopment. Thus Eder says: 7° “ Chemical development is character- 
ized by a reduction process, in which exposed silver halide is con- 
verted into metallic silver ; the unexposed is, however, left intact.” In 
1907, Dr. Homolka investigated the oxidizing powers of the latent 
images on organic compounds, other than the common developing 
agents, in order to determine if the latent image was an oxidizer in the 
widest sense of the term.?* After an examination of several com- 
pounds Dr. Homolka settled on indoxyl, which is an intermediate 
product between indol and indigo. Indoxy] dissolves freely in water 
and the resulting solution is completely and instantaneously oxidized 
to indigo by the mildest oxidizer. 

If.an exposed plate is placed in a two per cent solution of indoxyl 
a visible image develops in from five to ten minutes. This image can 

22 Brit. J. Phot., 1920, 67, 447, 469. 


23 Ausfiihrliches Handbuch der Photographie, 5th Ed., vol. III, p. 289. 
24 Brit. J. Phot., 1907, 54, 136. 


ee ee ee Se ee ae ee a Oe, ee ee ee 54 


eh 


“sy Pe ~~ 


THE LATENT IMAGE 211 


be shown to consist of two separate images, one of metallic silver and 
another of indigo, either of which may be separated from the other. 
A solution of potassium cyanide will remove the silver image leaving 
behind the indigo image, or a solution of sodium hydro-sulphite 
(Na,S,O0,) will remove the indigo image without affecting the other. 
Since indoxyl is reduced to indigo in the presence of the latent image, 
the latent image may be considered as an oxidizer. 

The latent image formed by light is evidently different in some 
way from that formed by stannous chloride, for as Dr. Homolka 
states indoxyl is reduced to indigo by the one but is unaffected by the 
other.2° A plate bathed in a 1: 200,000 solution of stannous chloride, 
and washed for an hour in running water, develops rapidly in indoxy]l, 
but an examination after fixing and washing shows that only a silver 
image has been formed and that there is no trace of ari indigo image 
as is the case with a latent image which has been formed by light. 

Homolka found that with exposures sufficient to produce reversal, 
only the silver image was reversed, the indigo image remaining nega- 
tive.*"° Crowther, however, disputes this statement and maintains 
that both images are reversed to approximately the same extent.?’ 

Action of Oxidizing and Halogenizing Agents on the Latent Image. 
—The latent image is either partially or wholly destroyed by oxidiz- 
ing or halogenizing agents. Thus, if a latent image is treated in a 5 
per cent solution of potassium cyanide (KCN) the image is com- 
pletely destroyed and development either physically or chemically is 
impossible. Substances which readily give up halogen, as ferric 
chloride, FeCl,, cupric chloride, CuCl,, mercuric chloride, HgCl,, 
and the halogen acids, as HCl, HBr, and HI, act in a similar man- 
ner. Strong oxidizing agents as ammonium persulphate, K,S,O,, and 
potassium permanganate, KMnOQ,, in an acid solution also depress the 
latent image. 

Theories of the Latent Image.—Theories of the latent image may 
be divided into two classes, physical and chemical; the one considering 
the change in a silver haloid on exposure to light to be a physical 
modification, while the other believes that a chemical change has taken 
place. 

The chief chemical theories are three in number: 

25 Brit. J. Phot., 1907, 54, 210. 


°6 Brit. J. Phot., 1907, 54, 267. 
~ “hot. J., 1915, 55, 186. 


212 PHOTOGRAPHY 


1. The oxy-halide theory, 
2. The sub-halide theory, 
3. The metallic silver theory (Silberkeim theorie). 


The leading physical theories are also three in number; viz., 


1. The molecular strain theory, 
2. The electron theory, 
3. The colloidal silver theory. 


Each of these, together with its supporting evidence and the chief 
objections to its general acceptance, will be considered as briefly as 
possible. 

The Oxy-Halide Theory.—Many things seem to indicate that the 
presence of oxygen is essential to the darkening of silver chloride by 
light, a fact which seems to have been distinctly stated first by Robert 
Hunt in his Researches on Light.”® 

Abney found that silver chloride remains unchanged for several 
months when exposed to light ina vacuum. A similar conclusion was 
reached by Carey Lea. In an experiment of Tugolessow 7® in which 
silver chloride was exposed to light in presence of stannous chloride, 
which is a strong reducing agent and would therefore tend to prevent 
oxidation, it was found that silver chloride did not darken, even if ex- 
posed to the action of strong light for several days. 

The opposite effect is observed when an oxidizing agent, as hydrogen 
peroxide, is used instead of a reducing agent; the silver chloride then 
darkens more rapidly and completely than in either air or water. 

The fact that the darkened silver chloride contains oxygen may be 
regarded as fairly certain from the work of Hodgkinson and Baker. 
Dr. Hodgkinson examined the darkened product and as a result of 
his analysis gave it the formula Ag,OCI,. Baker also found that the 
darkened product contained oxygen and settled upon the formula 
Ag,OCL. 

The equation for the action of light on silver chloride would there- 
fore be: ; 


8AgCl + O, == 2Ag,O0Cl, + 2Cl, 
or : 
8AgCl + 2H,O = 2Ag,O0Cl, + 4HCl. 


28 Second Edition, p. 80. 
29 Phot. Korr., 40 (1903), 584; also J. Phys. Chem., 1911, 15, 331. 


THE LATENT IMAGE 213 


Baker’s equation would be: 


4AgCl + O, = 2Ag,OCl + Cl, 
or 


4AgCl + 2H,O = 2Ag,OCl + 2HCl. 


Since silver chloride does not darken visibly when exposed to light, 
provided it is protected from oxygen, while the addition of an oxidiz- 
ing agent accelerates the rate of darkening, and the presence of oxygen 
in the darkened product can be shown, it would appear that oxygen 
is essential to the darkening action. If we assume that the latent in- 
visible image is different only in degree, and not in kind, from that 
which is visible, and which is formed by the continued action -of light, 
would it not be both possible and probable for the latent image to con- 
sist of a combination of the silver halide with oxygen? Such a theory 
has been brought forward several times by a few investigators but has 
failed to elicit much response from the scientific world at large. 

It will be observed that all experimental evidence in support of the 
theory has been obtained with pure silver chloride, and that all deter- 
minations which indicate the presence of oxygen have been made on a 
product which has darkened wisibly. This is an entirely different 
state of affairs from the latent image with which we are concerned, 
for in practice we have to deal not with a pure silver halide, but with 
a gelatino-silver halide complex, nor do we deal with a visible darken- 
ing but an invisible one which is only rendered visible upon the ap- 
plication of certain reducing agents known as developers. If we as- 
sume, with Tugolessow,® that the difference in the visible and invisible 
images is due primarily to a difference in the degree of oxidation, we 
are faced with the fact that one of two unlikely things must happen. 
First, it seems decidedly improbable that the combination of oxygen 
and silver halide can take place completely in the short space of an 
instantaneous exposure. Second, if the action of the light is catalytic, 
that is to say if the function of the light is to so alter the silver halide 
that it is subject to oxidation, which need not necessarily take place 
during exposure, then the change must be physical rather than chemical. 

In the opinion of the writer, the first of these theories is decidedly 
improbable, while the second, if carried to its logical conclusion, is es- 
sentially a physical theory in which the change may consist of either 
molecular strain or the emission of an electron, the existence of either 
being in itself sufficient to account for the latent image without the 
introduction of the oxidation factor. 


30 Phot. Korr. (1903), 40, 584. 


214 PHOTOGRAPHY 


The Sub-Halide Theory.—Carl Wilhelm Scheele laid the basis of 
the sub-halide theory in 1776, when he proved that chlorine is liberated 
when silver chloride is exposed to light under water. However, ac- 
cording to Waterhouse,*! Berthollet was the first to suggest the for- 
mation of a sub-halide containing less halogen than the normal salt. 
The sub-halide is thus the oldest theory of the latent image and it is 
likewise one which has at some time been supported by ds every 
authority of eminence. 

The existence of a single definite sub-halide has been supported by 
a number of authorities, notably Sir William Abney. According to 
this opinion the action of light on a silver bromide plate would be 


Ag,Br, = —= Ag,Br + Br. 


It is difficult to explain the phenomena of solarization on the as- 
sumption of only one compound, regardless of what formula be as- 
signed to it, consequently Trivelli brought forward the idea of several 
sub-halides Ag,Br,, Ag,Br,, Ag,Br,, Ag,Br,. None of these com- 
pounds have been prepared, and as there is no satisfactory evidence 
of their existence, Trivelli’s hypothesis has not been widely accepted. 

While experimenting with indoxyl, Homolka found that the oxidiz- 
ing properties of the latent image were greater than those of silver 
bromide itself. He therefore concluded that the latent image could 
not be composed entirely of sub-halide, otherwise how could its oxidiz- 
ing powers be greater than silver bromide which contains more halo- 
gen’ In his opinion, there is, in addition to any sub-halide which may 
be formed by the light, a per-halide containing more halogen than the 
normal salt. The fact that a latent image formed by light is able to 
oxidize indoxyl, while that formed by the action of stannous chloride 
can not do so, was explained by Homolka as being due to ait existence 
of per-halide in one case and not in the other.* 

Carey Lea,** Luppo-Cramer, and Reinders *! believe that the latent 
image is a phase of variable composition with silver bromide as the 
end term. There has been a good deal of discussion as to the con- 
stituents of the phase, some claiming that they are silver bromide and 
silver and others silver bromide and some sub-bromide. As we have 
no way of distinguishing between these two. hypotheses, this matter is 
as yet unsettled. 

31 Phot. J., 1903, 47, 59. 

32 Brit. J. Phot., 1907, 54, 136, 216, 267. 


33 Carey Lea, Amer. Jour. Sci. (3), 33, 349 (1877). 
34 Reinders, Zeit. Phys. Chem. (1911), 77, 363. 


ee Se oe ge a eee ee Cee ee ee eee ee en 


i ee i me . 
ee : 


~oyn ata us Soy rk eee en ) Pinas ee —_ 


THE LATENT IMAGE 215 


Evidences for the Liberation of Halogen.—lIf silver bromide is re- 
duced to a sub-bromide, containing less halogen than the normal bro- 
mide, bromine must be liberated and it should be possible to determine 
the presence of free bromine by chemical means. Abney mentions the 
fact that in the case of silver bromide long exposure to light produces 
an odor characteristic of bromine and that chemical tests indicate its 
presence. Scheele had previously shown that chlorine is liberated 
when silver chloride is exposed to light under water because chemical 
tests indicate the presence of chlorine in the water. The following is 
less conclusive but affords indirect evidence that there is a liberation 
of halogen: “ If the existence of sub-halides is assumed, it must neces- 
sarily follow that, if we restore to the sub-halide the bromine which 
has been lost, normal bromide must be reconstituted. As silver bro- 
mide may be formed by simply adding a solution of potassium bromide 
to silver nitrate, let us ascertain if an exposed film of silver bromide 
may be reconverted to normal bromide in the same manner. The ex- 
periment confirms the assumption. The impression formed by light 
disappears completely and the image cannot possibly be developed.” *° 

The student will please note that owing to other undetermined fac- 


tors the last experiment may not be entirely conclusive, and subject to 


different interpretations, while the other facts differ from the true 
latent image in that they are the result of prolonged exposure to light 
and also do not consider the possible consequences of any influence on 
the part of the colloid medium in which the grains of silver bromide 
are imbedded. The real question is whether there is a liberation of 
halogen under the conditions of ordinary practice. At the present 
time no definite answer can be given. Any liberation of halogen which 
might possibly occur is so infinitesimally small that no chemical tests: 
of which we are at present aware will indicate its presence. 

Do Silver Sub-Halides Exist?—The ‘isolation of a definite sub- 
halide of silver in the laboratory would naturally lend considerable 
support to any theory of the latent image which calls for the reduction 
of silver halide into a sub-halide, and it is well that we find what 
success has attended the attempt to prepare sub-haloids in the labora- 
tory. Wetzlar, in 1828, claimed to have obtained silver sub-chloride 
by treating solutions of ferric and cupric chloride with silver leaf. In 
1839 Wohler obtained a product which gave on analysis Ag,O by pass- 
ing hydrogen over heated silver citrate. This would be a sub-oxide, 
corresponding to the sub-haloid Ag,Cl. Von Bibra *° isolated a body, 

35 Mercator, “ Nascent Silver and Sub-Haloid Theories,” Brit. J. Phot., 1899, 


46, 620. 


36 Jour. fur Prak. Chem. (2), 12-55. 


216 PHOTOGRAPHY 


Ag,Cl,, by treating Wohler’s sub-oxide with hydrochloric acid. The 
majority of scientists who have attempted to repeat the experiments 
did not meet with success. 

The existence of silver sub-fluoride is fairly certain from the work 
of Guntz who passed a current of electricity through a solution of 
silver fluoride using silver electrodes. Guntz also prepared a sub- 
stance which he considered to be silver sub-haloid by treating silver 
sub-fluoride with hydrochloric acid. Some authorities are satisfied 
with the evidence while others are not. Otto Vogel *” attempted to 
prepare silver sub-haloids by treating silver nitrate with cuprous chlo- 
ride, bromide or iodide. The analytical results agree very closely with 
the accepted formulas Ag,Cl,, Ag,Br,, Ag,I,. Waterhouse, however, 
considers it extremely doubtful that the sub-haloids can be prepared 
in this manner and is of the opinion that these compounds consist of 
finely divided silver and unaltered silver haloid. 

Thus the existence of a definite sub-haloid is still open to question, 
and as late as 1911 Heyer, in attempting to confirm Luther’s earlier 
experiments which pointed to the existence of Ag,Cl and Ag,Br, was 
unable to find any conclusive evidence of their existence.** 

Objections to the Sub-Haloid Theory.—The principal objections to 
the sub-haloid theory are three in number: 


1. Lack of evidence for the liberation of halogen with short exposures. 

2. The questionable evidence for the existence of silver sub-halides. 

3. The improbability of chemical change resulting from the energy 
incident on the plate during exposure. 


The first objection has already been raised in a preceding paragraph, 
while the second has also been suggested directly above. There is 
absolutely no evidence at present that there is a liberation of halogen 
with the short exposures which form the latent image. We are forced 
to assume that what applies to the visible image also holds true for the 
latent image, an assumption which may or may not be true, although 
the similarity in the chemical reactions. of two images is in its favor. 

The only silver sub-haloid which can be said to exist is the sub- 
fluoride. The existence of Ag,Br,, Ag,Cl, can only be inferred from 
the existence of the sub-fluoride but it may be questioned if this evi- 
dence is sufficient. 

The third objection is that to which the opponents of the theory 
have attached the most importance. The amount of energy required 


87 Phot. Mitt., 36, 334. 
38 Heyer, J. Phys. Chem. (1911), 15, 557, 560. 


THE LATENT IMAGE 217 


to produce the latent image is exceedingly small, equalling, according 
to Dr. P. G. Nutting, about 10-' ergs, or possibly much less.*° 

It is difficult to accept as true a theory which calls for a chemical 
change when the available energy incident on the plate during exposure 
is so small. It seems very improbable that the small amount of light 
necessary to form the latent image could result in a complete chemical 
change. A chemical change may occur, but most investigators are of 
the opinion that this chemical change is preceded by a physical change. 
A physical theory can certainly afford a better explanation of the last 
named objection than can the sub-halide or any other chemical theory. 

The Metallic Silver Theory. (Nascent Silver or Silberkeim the- 
orie.)—According to the sub-halide theory, the effect of exposure to 
light on a silver halide is to cause some of the halogen to be liberated, 
resulting in the formation of a compound containing less halogen than 
the normal salt, or a sub-halide. The metallic silver theory differs 
from the sub-halide in that the liberation of halogen is assumed to be 
complete ; that is to say the halogen is entirely set free upon exposure 
resulting in pure metallic silver, the small grains of which act as a 
germ or nucleus in inducing development. 

According to Weisz,*° the metallic silver theory was first suggested 
by Scheele in 1777. Arago, in presenting the details of the daguerre- 
otype process to the French Academy of Sciences, again expressed the 
same opinion but the modern conception of the theory is due to 
Abegg ** and Oswald.*? 

In 1880, Eder found that upon touching an unexposed plate, im- 
mersed in a developer, with silver wire, reduction took place where 
there was contact between the wire and the surface of the plate. Ac- 
cording to Abegg metallic silver-is the cause of the extensive reduc- 
tion of unexposed silver halide contained in the film. It was soon 
proved, however, that reduction took place only when pressure was 
applied, and that any metal, or hard substance, even the corner of a 
sheet of paper, drawn across the sensitive surface, would cause reduc- 
tion in just the same way. Consequently the experiment fails to prove 
that metallic silver is the cause of the reduction, but on the contrary 


39 Since the above was written Sheppard and Wightman have investigated 
the subject. Their paper will be found in the Journal of the Optical Society 
of America for November, 1922, pp. 913-916. 

40 Zeit. Phys. Chem. (1906), 54, 311. 

41 Abegg, Archiv. Wiss. Phot., 1 (1899), 268 et seq. Also Brit. J. Phot., 1890, 
46, 1096. 


42 Oswald, Lehrbuch der Allgemeine chemie. 


218 | PHOTOGRAPHY 


shows that the image so formed is only an example of an artificial 
latent image due to pressure. 

_ Abegg was also able to show that the latent image was destroyed by 
nitric acid under certain conditions, and he brought this forward as a 
proof that the latent image consists of metallic silver because it is dis- 
solved by a silver solvent. This evidence, however, is inconclusive, 
since other acids and substances which do not dissolve silver are capable 
of destroying the latent image, while certain substances which destroy 
metallic silver are without effect on the image.** 

According to the metallic silver theory, the development of the latent 
image depends upon the presence of microscopic particles of metallic 
silver. Regarding this point Mees and Sheppard say: *4 

“Proceeding from the fact that a ‘germ’ of metallic silver will 
induce the deposition of further silver upon it from a super-saturated 
solution, it provides the most satisfactory theory of development, but 
fails to account for the resistance of the latent image to oxidizing solu- 
tions of such potential as destroy metallic silver.” However, it has 
not been definitely proved, to the writer’s knowledge at least, that the 
“germ” or nucleus which acts in promoting development cannot con- 
sist of some other substance than metallic silver, so while the explana- 
tion is satisfactory, it may not be the only interpretation. 

The original conception of the metallic silver theory has undergone 
modification during the past few years and many of those who formerly 
supported the theory in its original form are now of the opinion that 
the latent image consists of colloidal silver in an ultra-microscopic 
form. A further development of this idea is found in the colloidal 
silver theory of Renwick and others, to be treated later. 

The Molecular Strain Theory.—There are many scientists who deny 
that any chemical change takes place when a silver haloid is exposed 
to light to form the latent invisible image. They are ready to admit 
that chemical change may occur when the exposure is greatly pro- 
longed and a visible image is formed, but contend that the amount of 
energy present during the ordinary photographic exposure is insuffi- 
cient to produce photo-chemical decomposition. This has always been 
regarded as a weak point of the sub-halide, or for that matter any 
chemical theory of the latent image, and has caused many authorities 
to bring forward theories resting on a physical rather than a chemical 
basis. 


. 


43 Mercator, “ The Nascent Silver and Sub-haloid Theories,” Brit. J. Phot., 
1899, 46, 628. 
44 Investigations on the Theory of the Photographic Process, p. 199 (1907). 


THE LATENT IMAGE 219 


The molecular strain theory is essentially a physical theory. Ac- 
cording to this theory the action of the light sets up an internal strain 
within the molecule of silver bromide and causes the atoms to ‘pull 
apart from each other. The effect of the strain is to render the com- 
pound less stable, so that it is more easily reduced to metallic silver by 
the reducing agents known as developers. Fig. 154 is an attempt to 


@) QL Gelatine still 


under Strain 


ae ES Ss, oe : New Molecule 


Actual Photodecomposition WPS, eae Ag.Br 
2Aq Br, AqOCl, ete, 7 TN rt i 


‘s ‘ \ ‘ 
Absorbed —{o') ae er) H 
\ / 
by Sensitiser rv / Se 


FIG. -15§4. Cieratton of the Molecular Strain Theory of the Latent Image 
(Hasluck, The Book of Photography) 


show graphically what is supposed to take place upon exposure ac- 
cording to the molecular strain theory. In (1) the original unex- 
posed molecule of silver bromide consisting of silver and bromine 
atoms is shown. On exposure, the bromine atom begins to pull away 
from the silver atom (2) and as exposure is prolonged the strain is 
increased (3, 4,5). If exposure is sufficiently prolonged the molecule 
may be completely shattered and part of the bromine may be lost, re- 
sulting in the formation of a sub-halide, or oxidation Hay, take place, 
resulting in the formation of an oxy-halide. 

Evidence for the Molecular Strain Theory.—Photo-regression, or 
the relapse of the latent image with time, can be very easily explained 
by the molecular strain theory. The strain which has been set up 
between the atoms of the molecule of silver: bromide gradually les- 
sens with time until the point is reached where the strain ceases and 
the atoms again assume their proper places in the molecule of silver 
bromide. . 

The activity of a photographic plate at very low temperatures is 
_ also more easily explained by a physical than by a chemical theory. 


220 PHOTOGRAPHY 


Prof. Dewar **> found that at a temperature of —180° C. a highly 
active substance like potassium does not show any action when 
immersed in liquid oxygen, while the photographic plate was still 
fairly sensitive at a temperature of —200° Centigrade. It is rather 
difficult to see why light should produce a chemical change on such 
a relatively inactive silver halide at such a low temperature when 
no chemical change is produced with two active elements at a tem- 
perature twenty degrees higher. It is much easier to accept a physical 
change in the structure of the molecule than a chemical change, in 
face of this evidence. 

The remarkable stability of the latent image is often advanced as 
an argument for a chemical rather than a physical theory of the latent 
image, but is there any reason why a physically modified molecule of 
silver bromide should be less stable than a sub-halide? 

The Electron Theory of the Latent Image.—Like the theory of 
molecular strain the electron theory attempts to explain the latent 
image from a physical rather than from a chemical standpoint. It, 
perhaps, may be regarded as a logical development of the molecular 
strain theory, bringing it into conformity with the newer ideas of mat- 
ter and chemical action established by the theory of electrons. .Ac- 
cording to the electron theory, when light acts on a molecule of silver 
bromide, one, or possibly more, of the electrons composing the mole- 
cule are set free, leaving the molecule in an unstable condition which 
is readily reduced by the reducing agents known as developers. 

The Photo-Electric Effect—The action of light in setting free the 
electron with certain substances, or the photo-electric effect, has been 
rather extensively studied during the past few years, and advocates 
of the electron have repeatedly called attention to the bearing of this 
effect on the latent image.** 

It was ascertained by Hertz and his successors that light, especially 
of short wave-length, has a remarkable power of discharging nega- 
tive electrification from the surface of bodies and this effect can be 
proved to be due to the emission of negatively charged electrons. For 
instance a polished metal plate, when illuminated by ultra-violet light, 
is unable to retain a negative charge because of the discharge of the 
negatively charged electrons. It will, however, retain a positive charge. 
In addition to many metals, light can also liberate negatively charged 


45 Proc, Roy. Inst., vol. 13, p. 605. . 
46 See Photo-electricity, Allen; Photo-electricity, Hughes. 


ia 
2 ie 
ee es ae 


THE LATENT IMAGE 221 


electrons from the haloid salts of silver. As a matter of fact the 
silver haloids are vigorously photo-electric; their order to sensitive- 
ness being bromide, chloride, and iodide, which, it may be mentioned, 
is the order of their sensitiveness to light under identical conditions. 

Like the latent image the photo-electric effect is but little affected 
by temperature and the reaction may take place at the temperature 
of liquid air. The velocity with which the electrons are emitted de- 
pends upon the wave-length of the light: light of a short wave-length 
as ultra-violet produces a high velocity, while light of longer wave- 
lengths, as orange or red, produces a lower velocity, or the electrons 
may not be liberated, as a certain critical frequency is necessary to 
cause the electron to leave the atom. Compare these reactions with 
the sensitiveness of the photographic plate to light of various wave- 
lengths. Is not the similarity apparent? 

Evidence for and against the Electron Theory.—We know that 
ionization is easily produced by the X-ray tube and the rays of radium 
and as these act vigorously on the photographic plate it may be pos- 
sible for the latent image itself to be the product of ionization—or 
the splitting up of the molecule, or atom, into parts that are oppositely 
electrified. 

The relapse of the latent image with time can be satisfactorily ex- 
plained as being due to the gradual return of the electrons to the 
parent atom. 

The phenomena of reversal is explained by the electron theory in 
the following way: ** “If we suppose that, on the light continuing to 
act upon the silver halide grain, the electrons continue to be emitted, 
it follows that there will be a greater and greater accumulation of 
electrons surrounding the parent grain (of silver halide). The posi- 
tive charge on that grain will increase, and the negative charge sur- 
rounding it will increase also.*® If that goes on, a point will be 
reached at which neutralization takes place, this particular grain re- 
verting to its original condition.” With very long exposures chemical 
change very likely takes place and the change is no longer entirely 
a physical reaction. 7 

Chapman Jones, as well as others, objects to the explanation of re- 
versal because it seems illogical that a short exposure should cause 


47“ The Formation of the Latent Image on the Photographic Plate,” H. 
Stanley Allen, Phot. J., 1914, 54, 179. 
48 Because of the continuous liberation of negative electrons—Author. 


16 


222 PHOTOGRAPHY 


electrons to be liberated while a longer exposure causes the electron to 
return to the atom. It also appears that either the attraction between 
the silver bromide grain and the liberated electrons would increase 
until it balanced the separating force of light, and electrons would 
cease to be emitted; or the number of electrons returning to the 
parent atom would equal the number being liberated by light. In 
either of these cases we would have a state of equilibrium, and not a 
complete return of the liberated electrons to the original atom. Ac- 
cording to Dr. Allen ** this difficulty is removed when it is shown that 
before this stage is reached the accumulation of negative electrons 
outside the grain becomes so large that the insulating power of the 
gelatine, or dielectric, gives away and neutralization takes place. 
“That is, the force required to tear the electrons from the gel is, as a 
tule, less than that required to prevent the electrons from leaving the 
grain.” 

Our knowledge of the electron and its action is at present hardly 
sufficient to enable us to either accept or reject the electron theory. 
There is certainly a close similarity between the photo-electric effect 
and the latent image, while the liberation of negative electrons from 
the silver haloid on exposure to light seems to be a more satisfactory 
explanation of the conditions of exposure, and the small amount of 
energy required to produce the latent image, than any chemical theory 
could possibly be. | 

The Colloidal Silver Theory.—A theory which was apparently first 
advanced by Lorenz *® and one which has quite recently been ably 
seconded in a somewhat modified form by Renwick * is that known 
as the colloidal silver theory. According to this theory a sensitive 
emulsion is not composed, as many have thought, of grains of silver 
bromide, but of grains which contain, besides a trace of the gelatine 
in which they are imbedded, a highly unstable form of colloidal silver 
in solid solution and that on exposure to light this unstable form of 
colloidal silver first undergoes change. Renwick *? has shown that a 
solution of commercial collargol or positively charged colloidal silver 
has no effect on an emulsion regardless of the stage of preparation at 
which it is added. Luppo-Cramer ** has shown that if the colloidal 

49 Phot. J., 19014, 54, 184. 

50 Phot. Korr. (1901), 38, 166. 


51 Brit. J. Phot., 1920, 67, 447, 463. 
52 Ibid. 


53 Das latente Bild, 27, 29; Kolloid Chemie u. Phot., 99; Phot. Korr., 1907, — 


p. 484. 


. 


THE LATENT IMAGE 298 


silver has been previously coagulated, or rendered neutral, by an acid 
it is able to convert an emulsion directly to the developable stage 
without the necessity of exposure to light. According to the colloidal 
silver theory then the first action of light is photo-electric in character, 
consisting in the liberation of negative electrons and the production 
of the neutral form of colloidal silver, the presence of which is re- 
garded as the “ germ” necessary to induce development. 

The photosalts of Carey Lea have always been considered to have 
an important bearing on the nature of the latent image and have a 
close connection with the colloidal silver theory because the work of 
Reinders ** especially has proved that these photosalts, which their 
discoverer regarded as complexes of sub-halide and normal halide, 
are really solid solutions of. colloidal silver in silver halide. ‘“‘ One 
cannot fail to be impressed (says Renwick) with the extreme readi- 
ness with which silver in the colloidal, or even in the finely divided 
metallic state, goes into solid solution in a silver halide, and also by 
the great resistance offered by these solid solutions to the attack of 
silver solvents and to reduction by reducing agents.” 

It has long been known that ordinary gelatine emulsions may at 
times possess a slight red sensitiveness although undyed and since 
Eder has observed that colloidal silver has a color-sensitizing effect on 
silver chloride and bromide, it appears reasonable to assume that this 
red sénsitiveness is due to colloidal silver. 

Solarization is explained as being due to the formation of a rela- 
tively insensitive and undevelopable photo-halide by the interaction of 
the liberated bromine or hydrobromic acid on the latent image. 

The colloidal silver theory has the merit of reconciling the views 
of those who demand merely a physical change, those who require 
the formation of free silver, or a material chemically different from 
that originally present to act as a nucleus for development, and it 
should also meet with the approval of the exponents of the electron 
theory, since it is apparently well established that colloidal solutions 
of silver are negatively charged and are precipitated when the charge 
is lost. 

Sheppard’s Orientation Hypothesis of the Latent Image.—Until the 
discovery of the sensitivity centers and the general realization of the 
essentially disperse nature of the photographic emulsion, all theories 
of the latent image were tacitly based upon a homogeneous emulsion. 


54S. Physik. Chem., 191. 


224 ~ PHOTOGRAPHY 


The discovery of the sensitivity centers, their origin and distribution 
and the conception of photographic sensitive materials as aggregates 
of grains of silver halide of various sizes and sensitiveness has added 
to the problem of the latent image that of sensitivity. It is now gen- 
erally accepted that for a grain to be developable it must contain a 
nucleus. In other words the photo-effect, whatever its nature, is 
localized in the grain. The study of the latent image, therefore, be- 
comes a study of the mechanism of the reaction which takes place 
within the grain. 3 


The sensitivity centers were first conceived of as points of special — 


sensitiveness of a substance other than silver bromide on the grain 
of silver halide and facilitating catalytically the decomposition of the 
grain by light.°®> If the sensitivity centers consisted of specially sen- 
sitive points of a substance different from silver bromide, their spec- 
tral absorption should determine, or at least powerfully affect, the 
spectral sensitivity of the emulsion. In this case, the removal of the 
sensitivity centers by desensitizing in chromic acid would be accom- 
panied by a change in the wave-length sensitivity from that of the 
sensitivity centers to that cf the silver halide. Investigations on the 
spectral sensitivity of high speed emulsions before and after desensi- 
tization, however, shows that there is no appreciable change.®* This 
would indicate that the spectral sensitivity of the sensitivity centers 
does not differ to any observable extent from that of the silver halide; 
a conclusion which has been confirmed by the investigations of Toy 
and Edgerton ** on the number of developable centers produced in 
the silver grain by light of different wave-lengths, whereby they found 
that the number of centers produced was proportional to the light 
absorbed by silver bromide at that wave-length. 

It appears, therefore, that it is the silver halide and not the silver 
sulphide of the sensitivity centers that is the real photo-sensitive ma- 
terial. If the sensitivity centers then are not photo-sensitive and do 
not facilitate the decomposition of the grain by light, how can their 
undeniable influence on sensitiveness and developability be explained ? 

Sheppard °° suggests that the presence of the sensitivity centers of 
silver sulphide in the crystal lattice structure of the grain sets up a 

55 Clark, Brit. J. Phot., 1923, Dec. 14. Renwick, J. Soc. Chem. Ind., 1920, 39, 
156. Sheppard and Wightman, Science, 1923, 58, 80. 

56 Sheppard, Third Colloid Symposium Monograph. 

57 Phil. Mag., 1924, 48, 947. 

58 Third Colloid Symposium Monograph. 


ee ee Pee ey ee er 


ne ee ee ee ee a 


THE LATENT IMAGE 225 


deformation of the ions nearest the sensitivity center, the amount of 
this deformation decreasing as we leave the sensitivity center. The 
sensitivity centers act by concentrating the photo-effect so that the 
reduction of silver halide by light is localized around the sensitivity 
center, at which point the actual decomposition takes place. The 
first silver atoms formed as a result of this decomposition are sup- 
posed to act as a catalyst and facilitate the increase of the number 
of silver atoms deposited on the sensitivity center. 

In a paper on “ The Visible Decomposition of Silver Halide Grains 
by Light” (J. Phys. Chem., 1925, 29, 1568) Sheppard and Trivelli 
have shown that the visible decomposition of silver bromide proceeds 
in such a way as to indicate the existence of an orienting action. 
Whether the same case applies in the formation of the latent image, 
as implied in the Sheppard hypothesis, is yet to be shown. According 
to this hypothesis the sensitivity centers of silver sulphide do not play 
an active part in exposure. Their role is regarded as being purely 
passive and a consequence of the deformation which they set up in 
the lattice structure of the grain of silver halide. Sheppard has lately 
suggested, however, that the action of the silver sulphide centers may 
not be entirely passive and that the increased amount of light absorp- 
tion of silver sulphide, as compared with silver bromide, may result 
in “a certain amount of re-radiation, irradiated about the nucleus, 
and falling in the absorption region of silver bromide.” °° 

It is too early as yet, however, to do more than give the general out- 
lines of this the very latest hypothesis on the oldest and the funda- 
mental problem of photographic theory—the mechanism of the photo- 
chemical reaction and the formation of the latent image. 


BIBLIOGRAPHY 


GENERAL REFERENCE WorKS 


ANDRESEN—Das Latente Lichtbilde. 

Carey Lra—Kolloides Silber un die Photohaloide. (Translation by Luppo- 
Cramer, 2d Ed., 1921.) 

EpER AND VALENTA—Beitrage zur Photochemie und Spectralanalyse. 

Luppo-CrRAMER—Photographische Prob!eme. 

VALENTA—Photographische Chemie und Chemikalienkunde. 


_ $8 Brit. J. Phot., 1926, 73. - 


CHAPTER 1x 


SENSITOMETRY 


What is Sensitometry ?—The merest beginner soon realizes that ex- 
posure is by far the most important operation in picture making, and 
the one presenting the greatest difficulties on account of the variable 
factors which must be taken into consideration in calculating the 
proper duration of the exposure. One of the most important of 
these factors is the speed, or the sensitiveness, of the plate to light. 
Methods by which the sensitiveness of plates are determined come 
under the heading of sensitometry. While sensitometry is concerned 
primarily with methods of speed determination, this is not its only 
value, for in determining the speed of the plate we learn a great deal 
concerning its characteristics and properties, so that we may define 
sensitometry, in its broadest sense, as the study of the reproduction of 
light and shade by sensitive materials. 

General Resume of Sensitometric Methods.—As early as 1848, 
Claudet devised an instrument, which was termed a “ photograph 
meter,” for determining the speed of the daguerreotype plate. This 
' instrument gave to various portions of a plate exposures which in- 
crease in geometrical progression ag I, 2, 4, 8, 16, etc. The shortest 
exposure producing a visible impression on the sensitive material is 
taken as a measure of the speed of that material. Thus if the light- 


est visible deposit on one plate is produced by an exposure of I0 . 


seconds, while the time required for another material is double this, 
or 20 seconds, the relative speeds of the two areas 1:2. This method 
of determining the speed of plates by reference to the lowest exposure 
which produces a visible deposit is termed the threshold or Schellen- 
wert method. While it obviously gives some idea of the relative 
sensitiveness of different materials to light, it is not very reliable, 
except where the mere shape of an object is desired, for the test in- 
dicates the minimum exposure required to produce a visible image 
and is in no sense a guide to the exposure necessary for the proper 
rendering of gradation. Moreover it is possible to considerably alter 
the results by variations in exposure and development. - 
226 


" ae 
——— ee, i a a 


a a ee oe 


oS eee ee ve 


SENSITOMETRY 227 


Such was the state of affairs when Hurter and Driffield, two British 
amateurs, began their classical researches on plate speed determination 
which resulted in 1890 in the system of sensitometric investigation 
named after them—the H. and D. system. Briefly the H. and D. sys- 
tem differs from the threshold method in that the speed of a sensi- 
tive material is determined from several densities rather than one 
and affords a better indication of the sensitiveness, properties and 
characteristics of the plate than can be secured from a single density. 
Further, the final result is not influenced to quite the same degree by 
variations in development, or other after treatment. It is hard to 
estimate the real importance of the work of Hurter and Driffield. 
Their work resulted in much more than merely a method of determin- 
ing the speeds of sensitive materials. It is hardly too much to say 
that it indicated for the first time the rationale of the photographic 
process and that a large part, if not the greater part, of our present 
conception of the theory of photography had its inception at the 
hands of Hurter and Driffield. 

Several workers, notably Mees and Sheppard, have repeated the 
work of Hurter and Driffield using improved apparatus, and while 
these workers have done much towards the development of the sys- 
tem, their conclusions have always been substantially in line with 
those of the earlier investigators. Although the H. and D. system 
is not perfect, it is the most comprehensive and accurate method of 
plate speed determination which we have and is in general use 
throughout the world. 

Instruments for Sensitometric Investigation.—In plate speed de- 
termination by the Hurter and Driffield system we need: first, a 
standard light source for exposing plates; second, an instrument, 
known as a sensitometer or exposure machine, for impressing a series 
of exposures in a definite ratio on different sections of the sensitive 
material ; and third, an apparatus for measuring the deposits obtained 
upon development of the exposed material. 

Standard Light Sources.—Although daylight is used for the ma- 
jority of photographic exposures, in plate speed testing it is necessary 
that all plates be exposed to a light of exactly the same strength in 
order that the speeds may be comparable, for which purpose day- 
light, on account of its variability, is not suitable. The principal re- 
quirements of a standard light source are that it should be reasonably 
constant in intensity over fairly long periods of time and that it can 
be easily reproduced whenever and wherever desired. 


228 PHOTOGRAPHY 


The other important essential is a spectral distribution, or color 
range, comparable to that of daylight. Given light of the proper 
color range it is possible by the employment of suitable screens to 
secure light of very nearly the same color as daylight. 

The English standard photometric candle was employed by Hurter 
and Driffield but on account of its variability later workers have pre- 
ferred other sources such as the Harcourt pentane lamp, the Hefner 
amyl-acetate lamp, acetylene as employed by Mees and Sheppard or 
incandescent electric sources fun at a constant voltage. While the 
amyl-acetate lamp is still employed in some quarters acetylene or in- 
candescent electric sources are now more generally employed, usually 
in connection with a light filter so as to reproduce daylight as nearly 
as possible. 

Sensitometers.—Sensitometers, or machines used for impressing the 
sensitive material with graded exposures of a definite ratio, may be 
divided into two classes: (1) those which vary time while keeping 
intensity constant and (2) those which vary intensity, keeping time 
constant. The former are known as time scales, the latter as intensity 
scales. 

The Chapman Jones plate speed tester (Fig. 155) is an example of 
an intensity scale. The squares numbered from I to 24 are filled 
with pigmented gelatine of increasing opacity so that each numbered 


Fic. 155. Chapman Jones Plate Speed Tester 


step represents a decrease in exposure to the sensitive material placed 
beneath as the square root of 2. The plate to be tested is placed be- 
hind this scale in a special plate holder and the whole exposed to the 
light of a standard candle at a distance of one meter (39.37 in.). The — 
Eder-Hecht scale is very similar and is extensively used in Germany 
and on the Continent generally. 


oan 


SENSITOMETRY 229 


The chief objection to intensity scales of this kind for accurate 
investigation is the failure of the sensitive plate to obey what is 
termed the Bunsen-Roscoe reciprocity law, according to which ex- 
posure is regarded as the product of time and intensity (JT), and 
one factor may replace the other. Abney and others have shown that 
this law does not hold, at least over a very wide range, and that, to 
produce a given effect, time is not the reciprocal of intensity nor vice 
versa. 

Time scales are realized most easily by the employment of a sector 
wheel. That of Hurter and Driffield (Fig. 156) contains nine aper- 


Fic. 156. H. and D. Sector Wheel and Exposing Apparatus 


tures, each angle being twice the preceding, so that the ratio of ex- 
posures is in geometrical progression. This revolving wheel is en- 
closed in a light-tight box carrying at one end the standard light and 
at the other, behind the sector, the sensitive material to be tested 
(Fig. 156). 

The objection to a sector wheel is that the exposure is intermittent 
rather than continuous and the photographic effect of an intermittent 


-exposure differs from a continuous exposure of the same length of 


time by an amount which depends upon the intermittency and the 
speed of the sensitive material. For this reason time scales producing 
a continuous exposure are to be preferred. Exposure machines of 
this class have been devised by L. A. Jones, G. I. Higson and others 
and are now extensively employed in commercial plate testing. 
Instruments for the Measurement of Density.—While it would be 
possible to determine the density of the photographic deposit by 
actually weighing the silver, such methods would be both tedious and 
inaccurate owing to the extremely small quantities involved, and it 
is both simpler and more accurate to determine the density optically. 


The densitometers, or photometers, used for this purpose may be re- 


230 PHOTOGRAPHY 


garded essentially as light-balances. They serve to bring together 
two beams of light in order that the eye may more accurately com- 
-pare their intensities. The H. and D. densitometer (Fig. 157) is of 
the bench type, based upon the law of inverse squares, the principle 


| 


a 


iz 


NAAANARAANYZ 


Fic. 157. H. and D. Densitometer 


of which is well known to students of elementary physics. The Bun- 
sen grease-spot serves as an indicator, equal illumination on both 
sides causing the grease-spot to vanish, thus indicating photometric 
balance. The grease-spot is shifted first one way and then the other 
until a balance has been obtained. Then the silver deposit to be 
measured is inserted in the path of one of the beams of light and the 
grease-spot indicator again shifted until a balance is secured. The 


SENSITOMETRY 231 


difference between the first and last reading is a measure of the 
opacity of the deposit. 

Instead of varying the distance between the light source we may 
weaken one of them in a measurable way by polarization. Polariza- 
tion photometers of the type devised by Hufner and Martens are well 
adapted to the measurement of photographic deposits and are ex- 
tensively employed. 

Instead of polarization we may weaken one of the light beams by 
the use of absorption material. Densitometers of this type employing 
calibrated neutral-black absorption wedges have been designed by nu- 
metfous workers and are especially suitable for preliminary work and 
for instructional purposes. 

It may be pointed out that the density of a photographic deposit is 
not a definite, unvarying amount but that it depends to a certain ex- 
tent on the method of measurement. The photographic deposit is not 
homogeneous, as assumed by Hurter and Driffield, but is a light-scat- 
tering medium and consequently the Lambert-Beer law of absorption 
(to be referred to later) does not hold. The subject has been com- 
pletely investigated by Callier, F. F. Renwick and F.C. Toy. It has 
been shown that densities measured by parallel rays differ markedly 
from those measured by scattered light secured by placing the deposit 
in contact with opal glass. Renwick has shown that even then the 
apparent density is reduced by inter reflection between the opal glass 
and the negative. 

Opacity-Transparency-Density.—Since we must continually make 
use of a number of terms having reference to the absorption of light 
by the developed silver deposit, it is well that we become familiar with 
the laws governing the absorption of light and the terms used in con- 
nection with the same. 

Opacity is the term applied to the resistance of a substance to the 
passage of light. In other words, it may be expressed as the light 
which must fall on one side of the substance in order that a light of 
unit intensity be transmitted. Mathematically, this may be expressed 
as 

T/Is, 
I being the incident and J, the transmitted light. 

Transparency is just the reverse of this, being a measure of the 
fraction of the incident light which passes through the substance, or 


Ie/i; 
I being the incident and J, the transmitted light as before. 


232 | PHOTOGRAPHY 


In 1890 Hurter and Driffield introduced the conception of density. 
This they termed the amount of light stopping substance in the de- 
posit and defined as the logarithm of the opacity or the — logarithm of 
the transparency. } 


D = log,, (opacity) ==log,, (/2/), 
D==— log,, (transparency) ==—log,, (J «/I). 


This conception of the density of a photographic deposit was based 
upon the Lambert-Beer law of absorption. Lambert’s law states that, 
in passing through equal layers of a material, equal proportions of the 
light which traverses them are absorbed. Mathematically then, if J 
is the intensity which penetrates the surface, and J, the amount which 
has escaped absorption at a depth of +, then 


2 a Mind 


where the constant k is the absorption-coefficient for that particular 
substance. This law suggests that absorption is a molecular effect, 
each molecule absorbing a definite amount of the light incident upon it. 

Now in solutions the number of molecules is proportional to the 
concentration. Therefore the total absorption of a solution depends 
upon the concentration and the thickness of the layer traversed by the 
beam of light. If m is the concentration, then the law for absorption 
in solutions would be expressed as 


This is known as Beer’s law. . From the above it follows that 


__ log J — log Ie 
ae mx 


k 


When logs are taken to base 10, k is called the absorption constant, or, 
as defined by Hurter and Driffield, the density. | 

Within certain limits density is proportional to the mass of silver 
per unit area, or D= pM, where D is the density, M the mass of 
silver and p a constant termed the photometric constant. For an area — 
of 100 square centimeters having a density of 1, Hurter and Driffield 
obtained a value for p of .o131 gram of metallic silver. Eder ob- 
tained .o103 and Sheppard and Mees .o1031. : 

Perhaps the relation of opacity, transparency and density may be 
made plainer with the aid of the following step wedge (Fig. 158). 
In this we have four sections of increasing density, each additional 


cee ek 


SENSITOMETRY 233 


density being due to the superimposition of an equal density. That is 
to say, in section I, we have no silver deposit; in section 2, a silver 
deposit of a definite value; in section 3, two such deposits and so to N 


Fic. 158. Illustrating the Relation of Opacity, Transparency and Density 


layers. The following table then shows the relation between the opaci- 
ties, transparencies and densities of the various sections. 


Licut oF INTENSITY J 


No. layers of silver deposit...... 0) I 2 é N 
3 

Wransparenoy GPAs: : (=) (=) : (=) (=) x 

I 3 3 3 3 
PIenepArenOyaties. See AE Lee. I I a ole: ai 

I 3 9 27 (3) 
UC Es es 2 ES gO ee I 3 9 27 (3)N 
LL pS AEE gi a er 0) 477 954 1.43 alate 


The first line gives the number of layers of silver deposit. The second 
line shows the transparency expressed as powers of the fraction which 
is the transparency of one film. The third line shows these multiplied 
out, and the fourth gives the inverse of these or the opacity while the 
last line gives the log to a base of 10, or what Hurter and Driffield 
term the densities. : 
Exposure and Development of the Sensitive Material for Speed 
Determination.—Before proceeding with the exposure of the sensitive 
material for the purposes of speed determination it is necessary to 
adopt a standard unit of exposure. The photographic effect of a 
given exposure depends upon three things: the intensity of the light 
source; its distance from the sensitive. material, and lastly the duration 
of the exposure. Hence the standard unit of exposure must concretely 
specify the unit intensity of the light source, its distance from the 
sensitive material, and the unit of exposure. The standard adopted 
by Hurter and Driffield and now accepted as an International standard 


234 PHOTOGRAPHY 


is the Candle-Meter-Second (C. M. S.) which means the exposure of 
the sensitive material for one second, to a light-source with an in- 
tensity equal to one candle power, at a distance one meter from the 
light. . 

The material to be tested is first cut into strips I x 4% inches, the 
strips of sensitive material being preferably cut from the center of a 
large specially coated sheet in order to avoid any irregularity in the 
thickness of the coating. Two of the strips are placed in a specially 
made plate holder which is placed in the exposing machine. Only a 
small section of each strip is exposed, the remainder being reserved as 
a “‘ fog strip’ which is used to determine the density due to the fog- 
ging of the emulsion. This fog density is then subtracted from the 
total density as obtained from the densitometer readings in order to 
get the true density due to the action of light on the sensitive material. 

The exposure complete, the strips are ready for development. For 
- accurate results in sensitometric work it is absolutely necessary that a 
thermostat be used in order that the strips may all be developed under 
identical conditions of temperature. It would require too much space 
for us to go into details regarding thermostats for this purpose and as 
an understanding of the same is unnecessary to the subject of sensi- 
tometry from the standpoint of the student we leave the subject, re- 
ferring those interested further to the sources contained in that por- 
tion of the bibliography at the end of the chapter. 

The following pyro-soda developer is considered the standard for 
plate speed testing; but it is no secret that its use is not universal, 
probably because some of the later and more powerful reducing agents 
are capable of giving somewhat higher speed numbers. 


Pyrogalfol ass. cs.de sees apacase po. v0) ale elkouse oats alee a 8 parts 
Sodium carbonate’ (cryst.)........ 0. sesnss See ele ne 40 parts 
Sodium sulphite (cryst.) occ 5. ¢ceos oc 6 «seb eee 40 parts 
Water to makes... sco. 5 dec seine win’ s-p 4 ok ate pee ene 1000 parts 


No bromide must be used in the developer used for plate speed de- 
termination. Bromide delays development in a particular way which 
will be explained in the chapter on the Theory of Development and 
prevents concordant speeds from being obtained. Practically it may 
be said to slow the plate, for with normal development the lower densi- 
ties are restrained and do not assume their full intensity ; with increas- 
ing development, however, the lower densities gradually gain until 
finally the result is almost the same as if no bromide had been added. 
The uncertainty, however, which accompanies its use makes it unde- 


| 
{ 
} 
: 


| 
| 
- 
| 
| 
| 


== 


aie a ee eee ge re es ee eee 


al a” Sean a 


SENSITOMETRY 235 


sirable for plate speed testing, valuable though it may be in practical 
work. 

In practice two strips are nearly always exposed side by side and 
one of these is developed for twice the time of the other, for a reason 
which will appear later. 

After fixing, washing and drying, the densities of the various por- 
tions of the strips are determined by measurement in the densitometer, 
every precaution being taken to eliminate all sources of error so as to 
obtain the most accurate measurements possible. A note of the vari- 
ous densities and the duration of the exposure which produced them 
is made as each density is measured and with this information we 
are in a position to see what has been the reaction of the plate to the 
various exposures. 

The Relation of Exposure and the Growth of Density.—A strip 
exposed in a sector sensitometer contains nine exposures, a range of. 
1:512. This range is amply sufficient for the purposes of speed de- 
termination, but as we wish, for purposes of demonstration, to in- 
vestigate the effect of increased exposure on the growth of density 
over an even wider range we will assume that a number of strips have 
been exposed in such a way that we have obtained a range of ex- 
posures from 1 C. M. S. to over half a million C. M.S. In the table 
below we have placed opposite this series of exposures the densities 
obtained by Hurter and Driffield in an actual test. (See H. and D. 
Memorial Volume, p. 103.) | | 


Exposures in C. M.S. Densities Difference 
ey Ck a Se WOOD heat oua ete eae a 
PT ee acy nae 100 EVR ar So Sees 100 
OS ge 1 ee Scene RAO Ua, Fae ie Maal ee Re 180 
OSCE A ee rae BONES oral cpuheeich rete SOR ¢ 160 
Se a eos’. 2 eg Oe et, Rr ROR: SO ee 215 
AMG SPs vino 00 LOAD actions sae Rae 225 
ee Gd es 5 Phe Sey san ie Re 405 
Re ee ON ik Ps Leo AP ean aie Ae tir pee D> 530 
Me ele iy PGs a A gate oe oe oes coe 415 
bh, Ae on a SSS eee ok eee 245 
ea) Oe 6 Sa ea IONS? vate away ic teeter 450 
TT ENG oo A a NEA POPS ARE. ee aS aa 130 
ENA el tas thle pales. oj Fs hiss GB 260- hae ta adiautan aces 165 
ES REG SE a eae 8 AGG ey eeu elles 125 
eRe OG ie Sots Gok Kd CE OE eS aah lee eho 103 
ek cas eek tk Ctl YER I Te ee ere 0.034 
Ry ok oe vin sn oe BT ns nee care eee 0.194 
PATO ed reas Seek Ss a Be os Neko a ae ig 0.162 
MEAN prado vie rch teed ack POMS: Fol a te un ee ee 0.208 
PUMA ORs sos cons CH ee Oe ALAS Saeed ate BUN ck Sore eG Aare 0.056 


236 PHOTOGRAPHY 


If you have had the patience to go through the above table carefully, 
as you should do, you will observe that at first every time the exposure 
is increased there is practically an equal increase in the density, finally 
the increase in density for each additional increase in exposure becomes 
practically a constant and the differences in the last column show no 
change, excepting of course that due to experimental errors, and finally 
the growth of density for each additional exposure begins to grow 
less and less until a point is reached where additional exposure de- 
creases rather than increases the total density. 

The Characteristic Curve.—It is rather hard to get these points 
clearly fixed in the mind when the results are set up in tabular form. 
It is much easier to comprehend this relation between exposure and 
density by graphic presentation. This might be done by plotting 
density against exposure but in practice the density is plotted against 


CE 
VET 
AEE 
CAE 
CAE 
AE 
LTTE 


01 2 4 8 163264 &C up To..... oe a ee 
Exposure Seconds 


Fic. 159. The Characteristic Curve 


3.0 


2.0 


Density 


1.0 


0 


the logarithm of the exposure instead. There are two reasons for 
this: (1) The range of the two variables is quite different, for while 
the densities run to about 3, the exposures range as high as a half- 
million C. M. S., so that no real information can be secured from a 
curve in which density is plotted directly against the exposure. (2) 
There is no simple law between exposure and density so that no part 
of the curve will be a straight line representing an equal addition of 


~~ <> 


SENSITOMETRY 237 


density for each increase in exposure. Accordingly the density is 
plotted against the Jogarithm of the exposure instead of against the 
exposure directly. 

The exposure-density relation when plotted out in this manner pro- 
duces what is termed the characteristic curve of the emulsion and 
takes the general form illustrated in Fig. 159. It represents graphi- 


Correct 


12 4 8 16 328C 
Exposure 


Fic. 160. Step Chart Illustrating the Theory of the Characteristic.Curve 


cally the growth of density with increased exposure and summarizes in 
a handy and tangible manner most of the physical characteristics of 
the sensitive material, so that once we have obtained the characteristic 
curve of any material we are in a position to predict, not only its 
speed, but its various other characteristics. 

The Significance of the Characteristic Curve.—In an effort to © 


‘bring home to the student in a still simpler manner the real significance 


of the characteristic curve we will attempt to explain the various 
relations with the aid of Fig. 160. In this the steps are supposed to 
represent the various exposures and their height the amount of the 
corresponding density. We have divided the entire curve into three 
divisions, the significance of which we will shortly explain. 

The first period is characterized by a rapid increase in density as 
the exposures increase, the increase in density being approximately 
proportionate to the increase in the exposure. The relation existing 
within this period is shown by the following results of Hurter and 
Driffield : | 
Peoocre 20 ©, WM. 5. (1)......-...5- PICHSIEY No pt aG rae ce eee: Relative, 1. 
Eaovmura 100 ©. M.S. (8)..........:. Denagity, £.005...5-... 2+. Relative, 8.4 


With increasing exposures we reach the second period in which the 


addition of density for each increase in exposure becomes to all in- 
17 


238 PHOTOGRAPHY 


tents and purposes a constant. Within the limits of this period, repre- 
sented by the straight line portion of the characteristic curve (Fig. 
159), each time the exposure is doubled there is an equal addition to 
the density. That is to say that while the exposures increase in geo- 
metrical progression the densities increase in arithmetical progression, 
or for example: 


EXDOSUTES. lua eae I 2 4 8 16 32 64 128 256 
Densities .......... o I 's 3 4 5 6 7 8 


This relation between exposure and density has a special significance 
of great importance which will appear later. 

Passing on to the third period it will be observed that the steps are 
growing less and less for each exposure and finally a point is reached 
where there is absolutely no increase in density, after which increased 
exposure results in a decrease in density. This last portion of the 
curve (not shown in Fig. 160) known as the period of reversal is of 
great theoretical importance but, as it is reached only with enormous 
exposures, it has no significance in practice and thus we leave it, re- 
ferring the student to the literature of the subject for further infor- 
mation. 

Inertia as an Inverse Measure of Speed. ‘aNttip the straight line 
portion of the characteristic curve is produced so as to cut the log E 


base line, as shown in Fig. 159, an exposure is indicated which was 


termed by Hurter and Driffield the inertia. The inertia is an inverse 
- measure of the speed of the plate: that is to say, a slow plate has a 


high inertia while a rapid plate has a low inertia. To obtain numbers 


which are a direct measure of speed, the value of the inertia, as deter- 
mined by a sensitometric test, is divided by a factor which in the case 
of a standard candle as used by Hurter and Driffield equals 34. A 
plate having an inertia of .54 will thus have an H. and D. speed of 


34 

54 

The precise significance of the inertia as a measure of speed is some- 

what difficult to define. The exposure which it represents is not the 

“threshold exposure” (the minimum exposure necessary to produce a 

measurable density) nor does it indicate the maximum exposure which 

will give proper rendering of the gradations of the subject, but an ex- 

posure somewhere between these extremes, and Hurter and Driffield 

claimed that practically it indicated the beginning of the Pe of ex- 
posure in which correct gradation is secured. 


cates 


SENSITOMETRY 239 


Variation of the Inertia—While the precise significance of the 
inertia is somewhat clouded Hurter and Driffield could have found no 
other point so stable and so little susceptible to variation from which 
to calculate the sensitiveness of sensitive materials. Both Hurter and 
Driffield and also Sheppard and Mees have shown that, provided the 
plate does not contain free bromide, the value of the inertia is un- 
affected by variations in the time of development. The value of the 
inertia is also unaffected by variation in the temperature of the de- 
veloping solution (except with developers of very low reducing energy 
as hydrochinon) or by variations in the concentration, or composition, 
of the developer. Hurter and Driffield also claimed that the inertia 
was constant for all reducing agents, but Mees and Sheppard were able 
to show that this was not strictly true. According to the results ob- 
tained by these investigators there are two general classes of sensitive 
material, one class gives practically identical values for the inertia re- 
gardless of the developing agent, while the other class gives a some- 
what lower value with ferrous oxalate than with organic developers 
such as pyro, metol, etc.® 

Although the inertia is constant with increasing time of development 
this is not true if the plate contains free bromide, or if the developing 
solution contains a soluble bromide. In this case there is a lateral shift 
of the curve towards the right with a consequent increase in the value 
of the inertia and lower sensitiveness. However if development is pro- 
longed the restraining action of a soluble bromide on development be- 
comes less and less and the inertia point gradually shifts to the left, 
finally reaching almost the same value as would have been secured had 
the developing solution been free from soluble bromide.* It is for 
this reason that all developers used for speed testing must not contain 
a soluble bromide, otherwise the speed of the plate will depend upon 
the degree of development and concordant readings will be difficult to 
obtain. 

1H. and D. Memorial Volume, pp. 119-120. Mees and Sheppard, Jnvestiga- 


tions, p. 282. Later investigations, however, indicate that this statement is open 
to question and is not definitely settled as was formerly believed. See Shep- 


-pard, Phot. J., 1926, 66, 190. 


2 Mees and Sheppard, Investigations, p. 283, also 173. 

3 Mees and Sheppard, Investigations, p. 284. 

# There is some question as regards this latter statement. Nuietz in the Theory 
of Development, the latest authoritative work on the subject, was unable to con- 
firm the previous statements of Hurter and Driffield and Mees and Sheppard. 


He remarks, however, that the results obtained were obscured in many cases by 


fog so that the conclusions may not be correct. 


240 | PHOTOGRAPHY 


Watkins’ Central Speed Method.—Mention should be made at this 
point of the “central speed method” of Mr. Alfred Watkins, the 
eminent English authority. In selecting the inertia point Messrs. 
Hurter and Driffield remarked that for several reasons it would be 
more satisfactory to take as a measure of speed the beginning of the 
straight line portion of the curve but the difficulties of determining 
this point with sufficient accuracy made its choice impractical. Mr. 
Watkins has worked out a perfectly practical method of obtaining, 
not the beginning of the straight line portion, but its middle point the 
value of which he uses as a measure of the speed of the sensitive ma- 
terial and terms the Watkins central speed.® In the words of Mr. 
Watkins, “ The central speed method does not indicate the smallest 
exposure which will give a visible image, nor the minimum which will 
give truthful rendering, but that giving the best results.” It should be 
noted that the calculation of the central speed by the Watkins method 
is not subject to error on account of fog to the same extent as the 
Hurter and Driffield method. It has never been widely adopted, how- 
ever, because it indicates much lower speeds than the H. and D. 
method besides having several theoretical objections of its own.*® 

Wedge Methods of Sensitometry.—Since the introduction of a 
simple method of manufacture by E. Goldberg in 1910, neutral tint 
wedges have been rather extensively used in photographic sensitometry. 

Luther’s method of obtaining the characteristic curve directly with- 
out troublesome calculations by means of graded neutral tint wedges 
is particularly ingenious. A square neutral-gray wedge of known 
gradation, increasing in density say from 0 to 6, or an intensity-range 
of transmitted light from 1 to 1,000,000, is taken and the plate to be 
examined is exposed behind this wedge to a standard light source and 
developed to a high contrast. When dry the negative is placed over 


the wedge used for exposure but at right angles to the same. When 


observed by transmitted light the characteristic curve is seen as a 
rather diffused line. To sharpen this line a print is made on a process 
_ plate which is developed to the limit in order to secure the maximum 
contrast and from this a print is made on vigorous gaslight paper, the 

5 Watkins, Photography, its Principles and Applications, p. 306. 

6 A new method of plate speed determination based upon the characteristic 
curve as obtained by H. and D. methods, but differing in the measurement of 
the sensitiveness, has been described by Nietz in Theory of Development, Chap- 
ter IV. This method has not at present found widespread application and for 


this reason we will not discuss it further but refer the student to the original 


source. 


SENSITOMETRY 241 


boundary being reduced, locally if necessary, with a ferricyanide re- 
ducer in order to obtain a clear, sharp-cut line. The resulting curves 
may be scaled by impressing on the transparency the necessary scales, 
one of the unit lines of the log intensity scale being made coincident 
with a position line on the plate for which the effective exposure is 
known. The characteristic curve of a sensitive emulsion, as deter- 
mined by the use of crossed wedges, is illustrated in Fig. 161. 


TRE 
Hee HEATHER 


Fic. 161. Characteristic Curve Secured by Crossed Wedges 


The Perfect Negative.—We have now described the manner in which 
the sensitiveness of sensitive materials is determined and this was 
the primary object of the researches of Hurter and Driffield, who 
are largely responsible for the method which we have just described. 
The most valuable work resulting from the sensitometric investiga- 
tions of Hurter and Driffield, however, has been the relation of the 
same to the reproduction of tonal values by the photographic process. 

The function of photographic processes is to reproduce as faith- 
fully as possible the shape and tones of natural objects. Accurate 
drawing is an optical concern and strictly speaking is only indirectly 
connected with photographic processes. The truthful reproduction of 
tone and gradation, however, is a function of.the sensitive material 
and is thus distinctly a part of the photographic process. It is along 
these lines that the work of Hurter and Driffield has been the most 
fruitful, for they were the first to show the conditions governing the 
reproduction of tone by sensitive materials and its limitations. 

A negative is said to be a perfect representation of the subject when 


242 PHOTOGRAPHY 


the opacities of its gradations are proportional to those parts of the 
subject which they represent. It must be distinctly understood that 
this does not imply that the opacities must be in direct proportion to 
the light intensities of the subject in order that the reproduction be 
correct; it is proportional and not direct proportionality which is 
necessary. Thus with a subject having a range of intensities from I 
to 64 all negatives having the following opacity-ratios would be cor- 
rect reproductions of the original, because in each case the relations 
between the various opacities and the COMES aa portions of the 
subject are the same. 


Light intensities of subject..... eS yiatie uiese I 2 4 S.- 56 ai G4 

Opacities .....0. ake Gee eee yy I 2 4 a (ee 3 
4 WA I 2 4 3° 16 
4 ie 


2 4 8 


The relation between the light intensities, opacities and transpar- 
encies of a perfect negative may be evident from the following: 


Light intensities of subject....... a I 2 4 8 16 32 
Opacities i545... Bice Sea I 2 4 8 16 32 
Transparencies :.:.0s80 ae I 1/2. thal Re 2G ese 


The Density-Exposure Relation and Correct Reproduction.—We 
have previously investigated the relation existing between exposure 
and density for the purposes of plate speed testing; we are now 
about to discover the relation which it has to the ie of tone re- 
production. 

When we perceive in nature a uniform transition from faa to 
light we may be sure that the intensity of the light increases more 
nearly in geometrical than.in arithmetical progression, for in the lat- 
ter case the transition from dark to light is abrupt and harsh. Con- 
sequently, since in most objects the light intensities increase in geo- 
metrical progression, the opacities of a negative which is a faithful 
reproduction of the subject must also increase in geometrical progres- 
sion, Density we have previously defined (page 232) as the loga- 
rithm of the opacity, hence with a series of opacities increasing in 
geometrical progression the densities increase in arithmetical progres- 
ston. This relation may perhaps be clearer from the following nu- 
merical example: 


Light intensities of subject... .i.. 200%. 5..0255 I 2 4 8 oat 2 ee" Ga 
Dessities AGcra: le ee = 4 pla SiGe d anegate kon ee ri, ing 3 4 5 6 2 


Opacities 2050. spa ica wi «thee tee ee 2 4 Bee ees 


Le sD 


w) Toe) ow Toa = te fas eT eh le oo 
- 


a a en ae ee ee ek ee ee) ep ae 
7 E 


SENSITOMETRY 243 


The mathematician calls each term of an arithmetic series which 
corresponds to any given term of a geometric series, the logarithm of 
that term; and the law which alone would produce absolutely true 
tones in photography would be expressed by saying that the quantity 
of silver reduced on the negative, or the density, is proportional to 
the logarithm of the light intensity. 

From our discussion of the characteristic curve (page 236) 
will be remembered that the curve is obtained by plotting density 
against the logarithm of the exposure. This curve is not a straight 
line, as would be the case if the densities increased in arithmetical 
order over the entire range of exposure, but on the contrary has an 
f shape which was divided into three portions, the lower concave 
portion, the straight line portion and the convex portion. 

Attention has also been called to the fact that in the lower concave 
portion the densities increase on the same order as the exposures, or 
in geometrical progression. The light transmitted is therefore in 
arithmetical progression, producing a harsh, abrupt transition from 
dark to light which is characteristic of under exposure. This period 
is therefore termed the period of under exposure. 

In the straight line portion of the curve it is evident that the den- 
sities increase as the logarithm of the exposure, or ° 


dD/d log,, E =constant. 


Since this is the condition which has been shown to be essential to 
proper reproduction, this period is termed the period of correct rep- 
resentation or the period of correct exposure. In the convex portion 
of the curve the densities increase in less than arithmetical progres- 
sion ; consequently, the proper separation of the separate exposures 1s 
not secured and the result is flat and lifeless. This period is termed 
the period of over exposure. | 

The period of reversal is without significance so far as the subject 
of tone reproduction is concerned. 

Latitude of Sensitive Materials——The capacity of a given sensitive 
material in the matter of tone rendering is therefore determined en- 
tirely by the length of its straight line portion. It is in this respect 
that sensitive materials differ widely. Plates to be used for por- 
traiture, and other work in which a long scale of tones must be ac- 
curately reproduced, must have a long straight line portion so that 
the whole range of light intensities can come within the straight line 


244 PHOTOGRAPHY 


portion of the curve of the sensitive material. Plates for commercial 
and other work of this nature where greater contrast is required. and 
where the subjects do not possess such a long range of light intensities 
do not have this long straight line portion with its accompanying 
power of exact reproduction. 

The length of the straight line portion of the characteristic curve 
represents what is commonly termed the latitude of the sensitive ma- 
terial. Latitude in exposure depends upon two things: . 

1. Upon the extent of the straight line portion of the sensitive ma- 
terial. 

2. The range of light intensities in the subject. 

Let us suppose a sensitive material having a long straight line por- 
tion capable of rendering a range of exposures from 1-64 (Fig. 162). 


aan awe eee eS ane eEer ewan ee ee es 


0 1 2 4: 8 “16°32. Ga ee eee 


Fic. 162. Latitude and the Characteristic Curve 


Now if we have a subject with a range of exposures from 1-16 (rep- 
resented by the arrows) it will be evident that the exposure may be 
increased four times without forcing any of the tones into the pe- 
riod of over exposure. However, if the range of light intensities in 
the subject is increased to I-32, then the exposure could be in- 
creased only twice without forcing some of the exposures beyond the 
straight line portion. In the first case the sensitive material would 
be said to have a latitude of exposure of 1-4; in the second case 1-2. 
Hence the latitude in exposure possessed by a given sensitive material 


SENSITOMETRY 245 


is a relative term depending upon the range of light intensities in th 

subject. | 

_ Development and the Reproduction of Contrast—While correct 
exposure is absolutely necessary for correct rendering, it alone is not 
sufficient, for the time of development also plays a part. It is the 
function of exposure to secure the proper relationship between the 


Density 
B 


l 2 4 8 16 — ete. 
’ Log Exposure 


Fic. 163. Development and Constant Density Ratios 


densities and the light intensities which produced them. The densities 
are, however, only a half-way step towards the realization of a per- 
fect negative. It will.be remembered that it is the opacities which 
must be proportional to light intensities which produced them. While 
development is without effect on the relationship of the densities, it 
does affect very markedly the ratio of the opacities, so that it follows 
that development is a very important factor in securing correct re- 
production. 

Constant Density Ratios.—The effect of the time of development 
on a series of densities may perhaps be made clear with the aid of 
Fig. 163. The two series of gradations represent two sensitometric 
strips which have received identical exposure but different times of 
development. Series A we will assume to have received 4 minutes 
development; series B 2 minutes. The equal rise in the steps of each 


246 PHOTOGRAPHY 


staircase indicates that the relationship of the densities is the same in 
both cases and consequently the density ratios are not altered by varia- 
tion in the time of development. ‘This is what is meant by the law of 
constant density ratios, which was first enunciated by Hurter and 
Driffield in 1890." 

In confirmation of the law of constant density ratios we reproduce 
the following experimental data from an investigation of Hurter and 
Driffield: 


Exposures 
I 2 4 8 
Dewaty: (4” development)............ 0.775 1,000 — 1.180 1.250 
Ratio of densities Din-.. v2 sess 1.0 1.29 1.52 1.61. 
Density2 (12’’ development)........... 1.260 1.660 1.96 2.08 
Ratio of densities dish: sue oes 1.0 1.31 1.55 1.65 
Ratio DiJDe ey eee ee 1.63 1.66 1.66 | 1.60 


Thus it is evident that, within the limits of experimental error, evi- 
dence supports the law of constant density ratios. Since the ratio of 
the densities is unaffected by the time of development, it is evident 
that the ratio is a function of the exposure and that unless the ex- 
posure has produced the proper relationship between the densities and 
the light-intensities which produced them correct reproduction is im- 
possible. 

An Important Difference.—But while the density ratios are unaltered 
by the time of development, the opacity ratios are, the effect of an in- 


creased time of development being to considerably increase the ratio — 


of the opacities. Upon re-examination of the two staircases of Fig. 
163 it will be observed that while the progression of the densities is 
the same in both cases, the amount by which the densities differ, in- 
dicated by the height of the individual steps, is considerable and that 
the total range of A is much greater than B. Numerical data, from 
an experiment of Hurter and Driffield, which shows how development 
affects the ratio of the opacities, without altering that of the densities, 
follows: 


7“ Photo-chemical Investigations,” J. Soc. Chem. Ind. (1890), 9. 


SENSITOMETRY 247 


I 2 3 4 = 
Exposure | Density Density Opacity Opacity 

Co Mas. ratio ratio 

Seti eveioped 4). eu. i. 1.25 .310 1.0 2.04 1.0 
2.5 520 1.67 se 1.62 
5-0 725 2.33 5-30 2.59 

Stine aJeveoped O.. .. eek... 1.25 530 1.0 3.38 1.0 
2.5 905 1.70 8.03 2537 
5.0 1.235 2.33 17.18 5.08 

Strip 3, Developed 12”......... 1.25 .695 1.0 4.95 1.0 
2.5 1.140 1.64 13.80 2.78 


It will be observed that all three strips received identical exposures, 
but varying times of development. Column 3 shows that the density 
ratios are practically identical in all three cases, indicating that the 
increased time of development is without effect on the density ratios. 
Column 5, however, shows that the opacity ratios have increased con- 
siderably with increased time of development. Thus in the first strip 
the ratio of the minimum and maximum opacities is I—2.59; in the 
second strip the ratio is 1-5.08; while in the third strip the ratio has 
increased to 1-8.51. 

Development and Contrast—We now see clearly the relation be- 
tween exposure and development and the part each plays in securing 
a faithful reproduction of the subject as it appears to our visual 
senses. Exposure is responsible for the proper relationship between 
the tones, while development determines the differences between the 
tones. The amount of this difference is determined solely by the dura- 
tion of development and constitutes what is termed the contrast. Con- 
trol in development is confined entirely to the length of time which 
the developer is allowed to act. The growth of no one density may 
be restrained or increased without affecting the others proportionately. 
Erroneous exposure cannot be corrected by any alteration whatsoever 
in development, for if the proper relationship between the densities 
has not been secured by giving the correct exposure, then no amount 
of development will supply that relationship which must exist between 
density and exposure for correct reproduction. 

Thus there is one, and only one, time of development which will 
give a technically perfect negative. The proper time of development 
for a technically perfect negative is that time of development which is 


248 PHOTOGRAPHY 


required to produce a series of opacities which are directly propor- 
tional to the light intensities which produced them. 

In practice, however, owing to the differences in the properties of 
- various printing media, it may be advisable either not to reach this 
exact proportionality or in other cases it may be advisable to exceed 
it in order that the visual appearance of the positive print may cor- 
rectly reproduce the original subject. It must be remembered that 
the negative is only a means to an end. It is the positive print which 
is the final result and regulation of the contrast of the negative to 
meet the requirements of the printing medium is not only proper but 
necessary. 

Gamma as a Measure of Contrast——Hence we require not only a 
means of measuring contrast after it is obtained but also a means of 
calculating the time of development required to reach any given 
stage of contrast. For this purpose use can hardly be made of the 
opacities on account of the mathematical complexity which controls 
their growth and therefore it is common to express the degree of 
contrast in terms of densities and log exposures, the units of the 
characteristic curve. To the degree of contrast expressed in terms 
of density and log exposure, Hurter and Driffield gave the term 
gamma. (y) which has since been generally adopted. 

Gamma is the ratio of the density range of the negative to the 
range of the logarithms of the exposures producing them. Or in 
other terms 


Difference in maximum and minimum densities of a given series 
Difference in the logarithms of the corresponding exposures 


or again 
pe D, ere dD, 
Y log E, — log E; 


where D, and D, are the minimum and maximum densities of the cor- 
responding exposures £, and £,. 

Aside from being an expression of the degree of contrast in the 
negative, gamma also expresses the relation between the contrast of 
the negative and the subject which it represents. If the value of 
gamma is less than one the contrast is less than that of the subject, 
while if it is more than one the contrast is greater than the subject, 
provided that in each case the range of exposures fall within the 
straight line portion of the characteristic curve. The application of 


SENSITOMETRY 249 


gamma as a measure of contrast holds only within the period of cor- 
rect exposure. Under-exposure produces the effect of high gamma, 
while over-exposure has the reverse effect, but in both cases the dif- 
ference in the densities is not proportional to the difference in the 
logarithms of the exposures and gamma as a measure of contrast fails 
to have any real significance. 

Gamma and the Characteristic Curve.—If we connect the various 
densities of the two staircases of Fig. 163 with a straight line, it is 
evident that the angle which this line makes with the log exposure 
base is greater the longer the time of development. In other words 
the longer the time of development, or the higher the value of gamma, 


— 
. 


Densities 


Fic. 164. The Geometry of Gamma. (Brown) 


the steeper the slope of the straight line portion to the base. The 
slope of the straight line portion of the curve, or the angle which it 
makes with the log exposure line, is thus a measure of the amount of 
difference between the densities, or, in other words, of gamma. 

This relation may be expressed in a very simple manner by means 
of a little geometry applied to the characteristic curve. 

By giving a plate two exposures denoted at A and B (Fig. 164) on 
the log exposure scale, we obtain densities denoted by the heights of 
the vertical lines AC and BD. The horizonal lines OA and OB, 
therefore, measure the log exposures in like terms. 

Now apply the formula which we have previously arrived at from 
our definition of gamma, that is: 


tart Es Cech ene . 
| Gane log Ei — log Ee 


250 PHOTOGRAPHY 


In the diagram draw CE parallel to the log exposure base line OB. 


Then gamma = ->—— 7 = 73 See 


Now this ratio DE/CE is one way of measuring the angle CDE 
or 6 (theta). It is the tangent of the angle 6 (theta), the ratio of the 
side (in any right angle triangle) opposite one of the other angles to 

the side connecting this opposite side to the angle: 


perpendicular 
base 


This tangent of the angle 6 (theta), or tan 8, as it is called, is equal 


the ratio of trigonometry. 


to gamma, for it is plain from the diagram that the angle DCE is 


equal to the angle CFA, which is the angle of the slope of the straight 
line portion of the characteristic curve.® 

The Calculation of Gamma.—lIt would be possible to measure the 
angle and find the value of its tangent in the published tables but 
there is a much simpler way of finding the value of tan @ by using 
the chart itself. 

From the point 100 on the log exposure scale draw a line (HG in 
Fig. 164) parallel to the straight line portion of the characteristic 
curve (CD in Fig. 164) until it intersects with a perpendicular drawn 
through the 1000 point on the log exposure scale (G in Fig. 164). It 
is clear that since HG is parallel to CD the angle KHG is also equal to 
6 and therefore tan KHG is equal to tan @ or gamma. Tan KHG, 
however, equals GK/HK which in turn equals GK/1 since the dif- 
ference between the log of 100 (= 2) and the log of 1000 (= 3) is 1. 

Therefore if we mark on the vertical line KG a scale which cor- 
responds with that on the opposite side of the chart, the point where 
the parallel line from H meets the scale indicates the gamma without 
any calculation at all. This method of calculating gamma was de- 
vised by Hurter and Driffield. 

A slightly different method but based upon the same mathematical 
principle is used by Mr. Alfred Watkins. A distance is measured off 
on the log exposure scale equal to 10 times the value of the inertia 
obtained by projecting the straight line portion of the curve until it 
intersects with the log exposure scale. At this point erect a perpen- 

8] am indebted to Mr. George E. Brown for the above method which is taken 
from his “ Hurter and Driffield Doctrine” in the British Journal of Photog- 
raphy, 1921, 68, 374. 


. 
a 
| 
) 
| 
| 


SENSITOMETRY 3 251 


dicular line to intersect with the characteristic curve. The density 
at the point of intersection is equal to gamma. Thus in Fig. 164 the 
value of the inertia is 0.3 and | 


10 X 0.3 = 3.0. 


Erecting at 3 a perpendicular to the log exposure scale it is found 
that this perpendicular intersects the characteristic curve at a density 
of about 0.8 and is identical with the value secured by the previous 
method. 

The value of gamma may also be calculated from any two densities 
qvithin the straight line portion of the curve from the formula 


Dei 
Oat low ie — log Ay 


The graphical methods, however, are much more convenient. 

Instruments have been devised by which gamma may be obtained 
without calculations or plotting of densities: consideration of. these, 
however, is beyond the scope of this work.® 

The time of development necessary to obtain any given gamma 
when the time of development for other values of gamma is known 
will be given later in the chapter on the theory of development. 

Gamma Infinity.——The amount of contrast, and therefore the value 
of gamma, since gamma is the numerical expression of contrast, in- 
creases with the time of development up to’a certain point; beyond 
this point no further increase occurs, in fact, after this stage has been 
reached, lengthened development reduces rather than increases the 
value of gamma owing to the intervention of fog, the effect of which 
is greater on the lower densities than on the higher. The maximum 
amount of contrast or, in other words, the highest gamma obtainable 
with any given material is termed gamma infinity (yo). 

The value of gamma infinity depends chiefly upon the sensitive 
material, although experimentally small variations are secured with 
different developing agents.1° High speed emulsions for portrait 
work have a low gamma infinity as a high degree of contrast is never 
required in portrait work: in fact material tending to give a high 
gamma would be a disadvantage. In commercial, landscape and 
general exterior work greater contrast is required and sensitive ma- 

9 See: Watkins, Phot. J. (1912), 52, 206; also Photography, Its Principles 


and Applications. Renwick, Phot. J. (1914), 54, 163. 
10 Nietz, Theory of Development, p. 102. 


252 PHOTOGRAPHY 


terials made for these purposes are made to develop to higher values 
of gamma infinity than those made for portrait work. The greatest 
contrast of all is secured with plates of the process type as used for 
copying line work in black and white where absolutely clear lines to- 
gether with a field of the greatest possible opacity is required. 

' Gamma infinity may be determined experimentally, but as it in- 
volves the measurement of very high densities and since these may be 
more or less affected by the fog produced on long development, the 
process is subject to large experimental errors and the values of 
gamma infinity are generally determined by calculation from lower 
values of gamma. It will be remembered that in exposing the sensi- 
tive material in the sensitometer two strips were exposed under 
identical conditions and that these strips were later developed under 
like conditions, the duration of development, however, varying as 
Tino | 

A method of calculating gamma infinity from the values of two 
sensitometric strips developed so that 


t, = 2t, 


was first worked out by Mees and Sheppard in 1903.4 From certain 
mathematical data based upon the velocity of development they calcu- 
lated the following expression of gamma infinity in terms of lower 
gamma : 


of a 
Yo" Te T= eth 


This formula, however, is not so simple as that of Renwick: ” 


vo aie 
* one(aryy) eye 

A graphical method of determining gamma infinity which avoids all 
calculation has recently been worked out by Renwick and will be 


found extremely convenient.?® 


11 Phot. J. (1903), 43, 190. 

12 Phot. J., 51, 213. eae 

18 Phot. J., 1923, 63, 331. For two other methods see: Renwick, Phot. J., 1914, 
54, 165-6. Krohn, Phot. J., 1914, 54, 166-7. 


| 
j 
: 
| 
. 
| 
= 
: 


i 


SENSITOMETRY Dia 288 


GENERAL REFERENCE WorKS 


EpER—Systeme der Sensitometrie des Plaques Photographiques. (French trans- 
lation by E. Belin, 1902.) 

EpER AND VALENTA—Beitrage zur Photochemie. 

Fercuson—Hurter and Driffield Memorial Volume. With excellent bibliog- 
raphy to 1920. 

MEEs AND SHEPPARD—On the Theory of the Photographic Process. 


18 


CHAPTER X 


THE EXPOSURE OF THE SENSITIVE MATERIAL 


The Problem.—The problem in the exposure of the sensitive ma- 
terial is to find that time of exposure which is necessary under the 
prevailing conditions of light, subject and diaphragm to produce for 
each tone in the subject a proportionate density in the negative, so that 
the densities representing the tones of the subject may all lie within 
the straight line portion of the characteristic curve. 


There are four factors which determine the correct time of ex- 


posure : 


. The intensity of the light. 

. The subject. 

. The speed of the lens se the diaphragm used). 
4. The sensitiveness of the plate or film. 


Ww NR 


Light Intensity and Exposure.—The intensity of natural light is de- 
termined by the time of day and time of year, by disturbances in the 
atmosphere and by latitude. 

Based upon investigations of Bunsen and Roscoe, Scott of Dublin 
in 1880 drew up a series of tables showing the variation in the in- 
tensity of daylight due to time of year, time of day and latitude. 

Assuming equal conditions the table on page 255, therefore, indicates 
relative exposures for different seasons and latitudes. 

The countries south of the equator have their maximum light value 
in December, instead of June, therefore, if the positions of the months 
in the above table are exactly transposed, the table will apply both 
to the Southern and Northern hemispheres. 


Atmosphere.—lf the intensity of sunlight was unaffected by the at- — 


mosphere and physical obstructions the simple table above would be 

an accurate guide to photographic exposures. But the intensity of 

sunlight is markedly affected by the presence of clouds or dust par- 

ticles in the air. Clouds at times may increase the intensity of sun- 

light by reflection, but more often they decrease its intensity. Such 

alteration is extremely difficult to estimate except by chemical means 
254 


THE EXPOSURE OF THE SENSITIVE MATERIAL 255 


VARIATION IN EXPOSURE FROM MoRNING UNTIL EVENING FoR DIFFERENT 
LATITUDES 


By R. de B. Adamson, B.Sc. 


British Journal Photographic Almanac 


Lati- 
tude 


June 
May, July 


April, Aug. 
Mar., Sept. 


60° 
Feb., Oct. 
Jan., Nov. 
December 


June 
May, July 


April, Aug. 
55° Mar., Sept. 


Feb., Oct. 
Jan., Nov. 
December 


June 
- May, July 


April, Aug. 
EO° Mar., Sept. 


Feb., Oct. 
Jan., Nov. 
December 


June 
May, July 


April, Aug. 
40° Mar., Sept. 


Feb., Oct. 
Jan., Nov. 
December 


June 
May, July 


April, Aug. 
30° Mar., Sept. 


Feb., Oct. 
Jan., Nov. 
December 


North Hemisphere 


tole bole 


QAR W He eH 


BOwN eee Se 
vie 


WWN SS SA 
nile 


Walco loo 


ee 
bol hole 


Pleo mleo woo 


MORNING 


December , 
Jan., Nov. 
Feb., Oct. 


South Hemisphere 


Mar., Sept. 
April, Aug. 
May, July 


June 


Sa ee ee LO 


NiR le 


is ee 


DW SS eS eS 


| 
| 


nll 
NR le lH 


WN Hee 
OCORNHHH 


bol bole bole 


Nile lene 


NDS SS ee 
bole bole Rol 


I 
I 
I 
2 
3 
4 
4 


AFTERNOON 


December 
Jan., Nov. 
Feb., Oct. 


Mar., Sept. 
April, Aug. 


May, July 
June 


December 
Jan., Nov. 
Feb., Oct. 


Mar., Sept. 
April, Aug. 


May, July 
June 

December 
»Jan., Nov. 
Feb., Oct. 


Mar., Sept. 
April, Aug. 


May, July 
June 


December 
Jan., Nov. 
Feb., Oct. 


Mar., Sept. 
April, Aug. 


May, July 


June 


256 : PHOTOGRAPHY 


and although the eye after experience may be able to approximately 
determine its visual intensity, it cannot estimate its actinic intensity 


and it is this with which we are concerned. Towards evening, when 


the sun approaches the horizon, there is a marked decrease in the ac- 
tinic power of the light, but the eye detects little, if any, difference and 
it is difficult to estimate exposures under these conditions. The fol- 
lowing will give an idea of the relative intensity of light under dif- 
ferent conditions of cloudiness, but is only approximate, as there are 
many degrees of cloudiness and the eye cannot readily estimate the 
extent to which the passage of actinic light is ne by the same. 


‘Intense light (best possible light)......... 9 aes seeeane “dt nae 

Bright diffused light (sun behind pm e but still brights dig ics 

Light clouds (shadows visible).......... « & seca bite. 0.a:k eae < 

Heavy clouds (no shadows)......... wee cae scat Vie eS 

Very :heavy: clouds. « o.4ceeeet Sg ahas ere (die 8 7 ae ...4-5 or more 


The Subject—The majority of the subjects in general photography 
may be divided into six classes: Sea and Sky, Sea Views and Ship- 


Fic. 165. Sea and Sky 


ping, Open Landscape, Average Landscape, Outdoor Portraits, In- 
teriors and Indoor Portraits. > 

Class I. Sea and Sky—A subject, such as Fig. 165, which con- 
sists of sea and sky, receives the maximum amount of light, since 
there are no obstructions of any kind, while at the same time the 


7 
: 
4 
Z 
> 


] 
‘ 


THE EXPOSURE OF THE SENSITIVE MATERIAL 257 


amount and color of the reflected light is high, as few subjects reflect 
so large a proportion of the incident light as water the blue color 


Fic. 166. Sea View and Shipping 


of which is decidely actinic. The degree of contrast is low, since 
there are seldom any deep shadows near the camera and, therefore, 


Sa ee ee ee ee ee ee ee, ee ee a ee a ee 


Fic. 167. Open Landscape 


: lengthened exposure is not necessary to soften extremes of contrast. 
Unit Factor 1. 

; Class II, Sea Views and Shipping.—Should the subject contain 
vessels within one hundred feet, the exposure would have to be in- 


f, " a 


258° PHOTOGRAPHY 


creased on account of the near presence of a deeper shadow than 
common. Snow scenes, which contain no near dark objects and 
panoramic views, require about the same exposure as sea views with 


shipping. This class requires about double the exposure of the pre- 


ceding. Unit Factor 2. 
Class III. Open Landscape——Open fields and landscapes contain- 
ing no objects of importance within a hundred feet require about 


Fig. 168. Average Landscape 


twice the exposure of the class above. Such a subject is shown in 
Fig. 167. Factor 4. 

Class IV. Average Landscape.—li the figures in the above land- 
scape should be brought nearer the camera than one hundred feet, 
the exposure would have to be increased as less light will be re- 
flected from the subjects. Subjects in which the principal objects, 
whether persons, animals, or bushes, are about twenty-five feet from 
the camera fall into Class IV, illustrated in Fig. 168, and require about 
six times the exposure of Class I. Factor 6. 

Street scenes require about the same exposure as Class IV, if the 


—— se eee, eee 
7 


THE EXPOSURE OF THE SENSITIVE MATERIAL 259 


buildings are not close together or high, and if both sides are in sun- 
light. If the buildings are high, as in most business streets in the 
larger cities, the exposure must be increased several times. 

Class V. Outdoor Portraits——Portraits in the shade, as Fig. 169, 
require from 8 to 10 times the exposure of Class I (SEA AND 
SKY). Conditions in this class of work vary to such extremes that 
it is difficult to fix a factor, but that given will serve as a guide. 


Fic. 169. Outdoor Portrait 


Class VI. Interiors and Indoor Portraits—For the same reason, 
it is almost impossible to fix a factor for indoor portraits or interiors. 
Perhaps the factors of fifteen and twenty respectively will fit average 
conditions. 

Under equal conditions of light and color, exposure is unaffected by 
the distance of the subject and in a clear atmosphere, such as Switzer- 
jand or our own West, there are times when all objects beyond 
twenty-four times the focal length of the lens require the same ex- 
posure. In most parts of the country, however, there is a blue haze in 


260 PHOTOGRAPHY 


the air which possesses high actinic value and consequently shortens 
the exposure required for distant objects. 

Summarizing the factors for the different classes of So bi: we 
have: 


Class I Sea and sky... 0056 as cile gs aks wen 0/ecs 00ers gee een I 
Class II Sea views with shipping. ........ «+s seen eee 2 
Class"Iii - Open’ landseapes: alee een Oe ce eee bon Pikes 4 
Class IV Average landscape. ......,:..+++s4 ene eeeee 6 
Class V _ Portraits in shade: 7.9.42.) sea 8-10 
Class VI. Indoor portraits, -..2..00 eee ste tean oP 10-15 
Class VII Interiors. <4... seis s sien. as + eulul oll eee ee 15-20 


Speed of Plate—Owing to the absence of any universal standard 
in sensitometric methods, the plate speeds of one manufacturer can- 
not be compared with those of another. At the present time the only 
reliable basis of speed comparison for the plates of different makers 
are the tables issued by the makers of the Watkins and the Wynne 
meters, the American Photography exposure tables and the Bur- 
roughs-Wellcome Handbook. These lists include practically all the 
plates on the market in English-speaking countries. 


TABLE SHOWING CORRESPONDENCE OF SYSTEMS OF PLATE SPEED DETERMINATION 


By L. P. Clerc, in Revue Francaise from British Journal of Photography, 1922, 


69, 200 
Scheiner Eder-Hecht H. & D. Relative 
I 42 9 I 
a 46 9 1.27 
3 48 12 1.62 
4 50 15 2:07 
5 53 19 2.64 
6 56 24 3.36 
4 58 31 4.28 
ie: 61 40 5.45 
9 64 50 6.95 
10 66 64 8.86 
II 68 82 115 
12 71 104 14.4 
13 74 133 18.3 
14 T¢ 170 23-4 
15 80 216 29.8 
16 82 276 37-9 
2 84 351 48.3 
18 86 448 61.6 
19 88 570 78.5 
20 90 727 100.0 


THE EXPOSURE OF THE SENSITIVE MATERIAL 261 


Another disturbing factor which fortunately is seldom sufficient 
to cause serious trouble is the variation in speed of different batches 
of the same brand of plate. 

While every possible means is taken to ensure the uniform speed 
of every batch of plates turned out, it is beyond human skill to secure 
complete uniformity and even with the best of attention and care to 
all processes, large variations in speed will now and then occur. 
Sometimes the variation may be as large as 50 per cent but this is 
unusual, although variations of Io per cent are not uncommon. It 
would be a gain in accuracy and a distinct advantage to the practical 
worker to know the actual speed, secured by a.laboratory test of each 
batch of emulsion, for each box of plates he uses. 

Speed of Lens.—In a former chapter it was stated that with the 
same lens the exposure required by any two stops was inversely as their 
areas. From an optical standpoint this is correct, but as Abney has 
pointed out, the effective chemical action of the two is not the same, 
and, therefore, so far as exposure is concerned the optical relation is 
only partly true.t_ Apparently the law holds good when rapid plates 
are in use and the greatest departures appear in the case of very slow 
plates. Using lantern plates, Abney determined that if ten. seconds 
was required to obtain a certain density at light of unit strength, 1440 
seconds was only sufficient to give one fourth the same density when 
the light strength was 1/144 of its original unit value. Fortunately 
this matter is of no account practically, since rapid plates are not 
affected to any noticeable degree and the latitude of modern plates 
balances any such tendency. However, from the scientific viewpoint, 
it is interesting. Further work of a similar nature was published by 
Abney in his Treatise on Photography.” 

Determination of the Time of Exposure.—There are three methods 
in common use by which photographers determine the proper time to 
expose: 

1. The empirical. 
2. By the use of tables. 
3. Exposure meters. 


The first method calls for but the briefest comment. The so-called 
gift of exposure which many photographers claim to possess does not 
exist. The ability to estimate the time of exposure under given con- 


1 Brit. Jour. Almanac, 1894, p. 600. 
® Treatise on Photography, toth Ed., pp. 391-405. 


262 PHOTOGRAPHY 


ditions by examination of the image on the ground-glass or other like 
means consists simply in the comparison of present conditions with 
past experiences and were it not for the remarkable latitude of sensi- 
tive materials such methods would end in failure. While it is possible 
for one as a result of extensive experience under certain conditions to 
estimate with a fair degree of accuracy the time of exposure under 
similar conditions, for most workers and especially for the beginner, 
the occasional worker or for one who works under varied conditions 
such methods are inaccurate and unreliable. 

Reliable tables or exposure scales are much more satisfactory and if 
properly used will yield a high percentage of printable negatives, but 
here again a certain amount of judgment is needed, which is only ob- 
tained by experience, in order to properly classify the character of 
light, whether intense, bright, cloudy-bright, etc. While one gains 
ability in this respect with experience, even the trained eye is by no 
means an accurate judge of the actinic intensity of light, so that tables 
and scales are only another step toward the solution of the problem. 

The only way to ensure success in exposure is by the use of exposure 
meters which actually measure the chemical activity of the light at the 
time of the exposure. 

Exposure Meters.—There are two general types of exposure 
meters: (1) the actinometer, which measures the chemical strength 
of the light by the darkening of sensitized paper and (2) visual meters, 
which determine the strength of the light by photometric methods. 

The two standard meters of the first class are the Watkins and the 
Wynne. Besides these there are several others, the Imperial, Photo- 
meter M. and V., Haka-Expometer, Metropose Michant, Steadman 
and Beck. The first named are made in watch form and both de- 


pend upon the darkening of sensitive paper to a standard tint which, 


however, differs in the two meters. In practice there is little to choose 
between them. The Wynne has a lighter standard tint and requires 
only one fourth the time to make a test of the light as the Watkins, 
but a separate quarter-tint dial can be obtained from the manufac- 
turers of the latter. In the Watkins meter, the stop is placed against 
the plate speed number and therefore the scales do not need to be ad- 
justed as long as the same plate, or stop, is in use. The Wynne indi- 
cates at the same time the exposure for all stops, but must be reset 


whenever there is a change in the light value. Properly used, there is 


no question as to the accuracy of either. 
The rule for the use of the meter is to: Test the light in the shadiest 


1 


ees ee ee 


’ 
% 
. 
7 
| 
} 


THE EXPOSURE OF THE SENSITIVE MATERIAL 263 


part of the subject in which full detail is required. Therefore, if the 
subject is an open field, take the direct sunlight; if under the shade of 
trees, take the strength of the light where the subject is seated. Hold 
the meter to face the light that falls on the subject, not to face the 
camera nor the subject itself. In most cases, hold the instrument to 
face the sky but where the main light does not come from the sky, 
hold the meter so as to face the main source. The time required for 
the paper to reach the standard tint may be measured by a watch, by 
a pendulum or may be counted. A\ll in all, the latter is to be preferred, 
but the worker must be able to count seconds accurately—a matter 
which is not difficult after a little practice with a watch. One of the 
best methods of timing seconds mentally is to repeat, audibly if neces- 
sary, some phrase which one can easily speak in a second, such as, for 
instance, one-thousand-and-one, one-thousand-and-two, etc. Most 
people’s seconds are half-seconds. The watch is satisfactory, but 
with the pendulum it is difficult to watch both the meter and the bob 
at the same time. The stop-watch meters are accurate but expensive. 
It is easier to judge the proper matching of the two tints if the instru- 
ment is held at arm’s length and the tint viewed through half-closed 
eyes. The important thing to observe, and the whole secret to the 
successful use of a meter, is the time required for the sensitive paper 
to reach the darkness of the standard tint. Color is not to be con- 
sidered. Having found the actinometer time, as it is called, it re- ° 
mains to set the scales and read off the proper exposure. Full direc- 
tions accompany each meter and the reader is referred to these for 
further details. 

Should over or under exposure occur consistently when all of the 
above precautions have been taken, it may be corrected by a change 
in the speed of the plate. Thus, if over exposure occurs using plate 
speed 180 for Seed L Ortho plates, use a higher speed, say 250, while 
a lower plate speed, say 130 or 90, would be necessary if under ex- 
posure occurs with 180. Once the plate speed which gives the re- 
sults desired has been found, it should be adhered to and used as the 
basis of all calculations. It is seldom necessary to make smaller altera- 
tions in plate speeds than 50 per cent. Thus a change from go to 100 
would not be noticed and 130 might be used without noticeable 
alteration. 

_ In the case of indoor portraits or interiors the time required for the 
determination of the actinometer time is lessened by the use of lighter 
tints for purposes of calculation. Thus with the Watkins meter the 


264 PHOTOGRALRE y. 


first visible darkening of the sensitive paper requires exactly 1/16 of 
the time necessary for the standard tint. One can, therefore, take the 
time in minutes or seconds for the sixteenth tint, multiplying this value 
by sixteen to obtain the full actinometer time. In the case of interiors 
or still life one may expose the plate and meter at the same time, the 
diaphragm employed being such that the camera exposure is equal to 
the actinometer exposure for either the sixteenth or quarter tint. 
Tables for this purpose are given in the instruction booklet accom- 
panying each meter. 

Corrections for Special Subjects——For all ordinary subjects, as 
open landscapes, average landscape views, trees, portraits in shade, 
buildings, groups and interiors, no correction is necessary and the ex- 
posures indicated by. the meter will be found about right. There are a 
few subjects, however, which require alterations in the meter reading 
because of their high actinic color and on account of exceptional re- 
flecting power. The following table gives the proper alteration to be 
made for the more important of these exceptional subjects: 


Sky or Sea and sky.) jigs sce tease a ee .... 1/10 indicated exposure 
Snow or glacier scenes, sea views with shipping, black 
and white prints... 05....VG..464 ¢00keas ete 1% indicated exposure 


Open landscapes, lake views, river banks from the 


water, copying half-tone photos..............++- 1% indicated exposure 


Very dark colored objects as old furniture and dark 
paintings in a non-actinic color.............0.005 1% indicated exposure 


Visual Meters—Types, Principle and Methods of Use.—Meters of 
this type measure the visual intensity of the light reflected from the 
shadows. Most of them are really extinction photometers and deter- 
mine the intensity of light reflected by finding the point at which detail 
in the shadows disappears. 

Meters of this type consist of two essential parts: 


(1) A filter to subdue the highly visible but non-actinic rays as yellow 
and green. : 

(2) A mathematically and scientifically accurate means of determin- 
ing the value of the light reflected from the subject. 


To be absolutely accurate the filter would have to be adjusted for the 
plate in use, but owing to the wide latitude of commercial plates, this 
has not been attempted and the meters of this type on the market have 
a screen which is suited only to non-orthochromatic plates. The most 
active rays chemically are the invisible violet, known as ultra-violet, 


ee eee ee ee ee ee eT Se 


THE EXPOSURE OF THE SENSITIVE MATERIAL 265 


and these it is impossible to measure visually so that in this respect a 
visual meter falls short of the standard set by an actinometer. Mathe- 
matically considered, the methods employed by the various meters are 
sufficiently accurate for practical use but there is a personal factor to 
be considered in the measurement of light intensities by the eye. The 
author has found that no three people out of a dozen among his stu- 
dents will obtain the same readings because to each the details in the 
shadows disappear at a different stage. He has made no tests as to 
the effect of changes of the size of the pupil of the eye during, or 
before, the examination but does not doubt that there is a wide vari- 
ance among different persons in this respect. For instance, a person 
passing from a darkened room into the sunlight and at once testing the 
light visually would be almost certain, other things being equal, to 
obtain a higher reading than one who had been outside in the sunlight 
for.a half-hour or more. It is this personal error—this variation of 
the size of the pupil under different conditions—that tends to destroy 
the accuracy of the instrument and render its readings fluctuating. 
The writer is aware that many use these meters with good success and 
that this may in some measure prove their value, but he is equally 
convinced that both scientifically and practically the actinometer is 
superior, and this conclusion has been reached not:only from his per- 
sonal experience but from large numbers of students, whom it has 
been his privilege to instruct. 

Detailed methods of use are included with each instrument and, as 


_ they vary considerably, the reader is referred to the descriptive matter 


issued by the firms manufacturing the same, rather than encumbering 
these pages with matter which may readily be obtained elsewhere. 
Prominent meters of this type for sale in this country are the Heyde, 
McMurty, Trilux, Diaphot and Justaphot. 


GENERAL REFERENCE WorKS 


BoursAuLtt—Calcul du temps depose en Photographie. 
CLrEMENT—Methode practique pour determiner exactment le temps depose en 
photographie. 
FrApriE—The Secret of Exposure. 
Outdoor Exposures, Photo-Miniature No. 54. 
Exposure Indoors, Photo-Miniature No. 157. 
STEADMAN—Unit Photography and Actinometry. 
Watxins—Manual of Exposure and Development. . 
Correct Exposure—How to Secure It, Photo-Miniature No. 105. 
Viwat—Calcul des temps de pose et Tables photometriques, 


i, i ees i a 


CHAPTER XI 


THE THEORY OF DEVELOPMENT 


Introduction.—Like nearly all photo-chemical reactions, develop- 
ment is a complex and many-sided process. It is neither entirely 
chemical, nor physical, nor physico-chemical, but is a composite of all 
three. The first step in the process of development is the diffusion of 
the developing solution through the gelatine which carries the exposed 
silver halide grains in suspension. This constitutes what is termed 
the invasion phase and is entirely a matter of physics, being controlled 
by the physical laws of diffusion. Once the developing solution has 
reached the silver grain which has been acted on by light a reaction 
takes place in which the exposed silver halide is converted to metallic 
silver. This stage is chemical in character and may be termed the 
reduction phase. ‘The silver so formed, we will find, is in solution and 
before the image is formed precipitation must take place. This is 
termed the precipitation phase and is chemical in character. The pre- 
cipitation of silver results in a density and the difference between the 
densities produced by the action of varying intensities of light produces 
contrast. The growth of density and the growth of contrast are con- 
trolled by both the physical and the chemical phases of development 
and hence these are physico-chemical in character. 


Thus we find that development may be broadly divided into three ~ 


divisions : 
1. The physical viewpoint. 
2. The chemical viewpoint. 
3. The physico-chemical viewpoint. 


We will accordingly investigate the theory of the subject in this order. 

The Invasion Phase.—The general properties of gelatine and the 
structure of the photographic emulsion were considered in the chapter 
on Emulsions, where we found that the photographic emulsion con- 
sists essentially of exceedingly minute particles of silver bromide held 
in colloidal suspension in gelatine. The exact structure of gelatine is 
still an unsettled matter, but it will suffice for our purposes if we rep- 
resent by Fig. 170 the structure of the gelatine in which the grains of 


silver bromide are imbedded. The structure is of course very ir-_ 


266 


| 
| 
: 
| 
3 
| 


THE THEORY OF DEVELOPMENT 267 


regular, probably there is no definite structure, but the illustration will 
serve to illustrate the physical conditions of development. From an 
examination of this figure it will be observed that a jelly consists of a 
large number of cells which are intersected in all directions by pas- 


ASS E EXE | WWF We we SVBEX Dd ‘« vic 
POMS So 
SRI OSES BS 

Ws w4N), > NZS [SE 
LESSOR: ee 


Fic. 170. The Invasion Phase of Development. (Mees) 


sages. The cells and passages contain a weak solution of jelly while 
the walls consist of a very much stronger film of gelatine, the whole 
resembling a sponge filled with water. In b of the figure we have 
- indicated by a black dot the grain of silver bromide in each of these 
cells of gelatine. Remembering that the individual grain is the limit- 
ing factor in development, we are now in a position to trace the course 
of a molecule of developing solution which is passing through the 
jelly on its way to the exposed grain of silver bromide. 

Beginning at the surface of the film, a molecule of the developing 
solution rapidly diffuses through the passages and arrives at the cell 
wall. Here the gelatine is more resistant and the penetration is less 
rapid. Once the molecule has passed through the cell wall, chemical 
reaction proceeds immediately. The rapid diffusion of the developing 
solution through the passages is termed macro-diffusion, while the 
much slower penetration of the cell wall, which ‘constitutes the second 
phase of the physical action in development, is termed micro-diffusion. 
The first phase is of exceedingly short duration and is complete within 
a very few seconds after the developer is applied. The second may 
require from several seconds to more than a minute. Both stages 
must be accomplished before the grain of exposed silver halide can be 
converted to metallic silver, and since this is necessary to produce the 
image, three stages of development, four in fact, have taken place when 
the image appears. Owing principally to the exhaustion of the de- 
veloper as it penetrates the depth of the film, the exposed grains 
which lie near the surface are the first to be reduced, while those which 


268 PHOTOGRAPHY 


are buried deeper within the film develop more slowly. Hence all 
three phases are taking place at the same time but in different parts of 
the film. 

The Chemical Reaction within the Cell—the Reduction Phase.— 
Development is essentially a process of chemical reduction. Accord- 
ing to the earliest theory of importance the process consisted in the 
reduction of the exposed silver bromide to metallic silver by the de- 
veloping agent, the liberated bromine combining with the alkali to 
form an alkaline bromide. This reaction may be represented by the 
following equation in which D represents the developing agent: 


AgBr + DNa— Ag + NaBr + D. 


While apparently satisfactory, this theory really explains very little. 
For instance, it offers no explanation of the manner in which the de- 
veloping agent is able to reduce the exposed silver halide to metallic 
silver. Accordingly later explanations are based on the theory of 
ions, which can explain more exactly the nature of the reaction which 
takes place. We know that chemical reaction can take place only in 
solution and the theory of solutions teaches us that a salt in solution ~ 
is split up into the so-called ions which are atoms of the elements — 
carrying an electric charge. Metallic or basic ions carry a positive 
(-+-) charge and are called cations, while acid ions carry a negative 
charge and are termed anions. Thus common salt (sodium chloride, 
NaCl) when dissolved in water is disassociated into the sodium cation 
and the chlorine anion. In the form of an equation this reads 


NaCl — Na* Cl- 
and in the case of AgBr this becomes 
AgBr — Ag? Br-. 


A salt so disassociated is termed tonized. 

According to the view most generally accepted in the scientific wentie 
of to-day, the first stage of the reduction phase consists of the disasso- 
ciation and ionization of the exposed silver halide and the developing — 
solution which has penetrated the cell wall and dissolved the silver 
bromide. As the two are both ionized there is an exchange of ions 
between the two. The silver cation receives an anion from the de- 
veloper which is sufficient to remove its positive charge and neutralize 
it. It then ceases to be an ion and becomes metallic silver. The 
bromide anion is fixed in the form of a metallic or organic bromide 


THE THEORY OF DEVELOPMENT 269 


according to the character of the developing agent. Owing to the ex- 
ceedingly complex nature of the organic developing agents and to the 
secondary reactions which take place it is difficult to be more exact on 
this point. The only reducing agent whose action may be said to be 
fully understood is ferrous oxalate, although hydrochinon follows a 
fairly simple reaction when used without a sulphite. The reaction 
with hydrochinon is as follows: 


_ AgBr — Agt — Br- 


eee — Nat 
©. 4 2Na0H> ae bee + HO 
—O-Nat 
Bee ci Ionized hydrochinon. 


—Nat 


ich +- 2Ag'° Br = can + 2Ag + 2Nat+Br— 


YY 
— Nat 


Metal 


The ionized hydrochinon loses two anions which unite with and neu- 
tralize the two silver cations forming metallic silver, the two oxygen 
ions combine to form quinone and the bromine anion unites with the 
sodium cation to form sodium bromide. This completes the first stage 
of the reduction process and constitutes the reduction phase. 

The Precipitation Phase——We may now picture to ourselves the 
second phase of the chemical reaction within the cell, known as the 
precipitation phase. The metallic silver formed is in colloidal solu- 
tion, and as the reaction proceeds more and more silver will be formed 
until the solution becomes saturated with respect to silver. The re- 
action must then stop, unless the silver is induced to precipitate. Some 
germ or nucleus is necessary in order to induce precipitation and the 
production of this nucleus is the function of the exposure. The sub- 
stance forming the latent image is thus the term which induces the 
silver to deposit and by so doing produces the image. “As silver is 
deposited, the concentration of silver solution within the cell is conse- 
quently lowered, and the reaction is increased, the deposited silver thus 
acting auto-catalytically (but only for the individual grain). The low 

19 


270 PHOTOGRAPHY 


solubility of silver is sufficient explanation of the localization of de- 
velopment to the individual grain.” + 
Investigation has shown that any trace of a nucleus is sufficient to 


render all of the silver bromide in that cell developable. Hence, pro- 


vided the cell has complete access to the developing solution, there is 
no partial development of any cell; it is either completely developed 
or not at all. 

Development as a Reveruibis Reaction.—The arrows in the above 
equation indicate that development from the chemical standpoint may 
be considered as a reversible reaction. This has been experimentally 
proven for the iron developer and for quinol. Mees and Sheppard ? 
have shown that a solution of potassium ferri-oxalate and potassium 
bromide act on a developed negative to produce silver bromide. With 
hydrochinon, quinone and potassium bromide act on an exposed and 
developed plate to form quinol and silver bromide. This reverse ac- 
tion is largely prevented by the presence of the alkali and sulphites, 


always used with organic developers, so that the first oxidation product - 


of the reducing agent is further oxidized by air and by the silver bro- 
mide and the reaction is then no longer reversible. 

The Action of Sulphites, Soluble Bromides and Alkali in Organic 
Developing Solutions.—The Action of Sulphites—Sodium sulphite 
is customarily added to all organic developing agents for the purpose 
of preserving the developer and preventing oxidation and consequent 
staining of the gelatine by the solution when in use. Notwithstanding 
its universal application its action is but little understood. There are 
four possible ways in which the sodium sulphite may aid in prevent- 
ing oxidation of the developing solution: 

1. The sulphite may be more readily oxidized that the developing 
agent. 

2. The reverse may be true; but the sulphite may regenmeaee oe de- 
veloping agent. 

3. The two may form a complex salt which is less subject to oxida- 
tion than either alone. 

4. There may be no protective action, but only a division of oxida- 
tion, half of the oxygen going to the sulphite and half to the developer. 
This, as pointed out by Bancroft, would mean an actual though not 
theoretical decrease in the rate of oxidation. 

The first we know definitely to be a fallacy, as many of the organic 

1 Mees, Phot. J., 1910, 50, 403. 

2 Zeit. wiss. Phot., 1904, II, 5. 


ee ee 


aie ot 


| 
] 


ee ee ee ee ee ee ee, lel ll oe 


THE THEORY OF DEVELOPMENT 271 


developing agents are more easily oxidized in solution than sodium 
sulphite. The experiments of Mees and Sheppard * with hydrochinon 
support the second explanation, but they were working under different 
conditions than those of actual practice. It is doubtful that any re- 
generation of the developing agent occurs with other developers than 
hydrochinon. There is at any rate no experimental evidence for any 
but hydrochinon at the present time. The third and fourth seem to 
be nearer the truth, for we know that hydrochinon and sulphite enter 
into combination and it is quite possible for the combination to be less 
subject to oxidation than either alone. But, like many other matters 
of everyday photographic practice, this is still an unsolved problem 
theoretically. 

The Action of Soluble Bromides——The addition of a soluble bro- 
mide slows development by diminishing the degree of ionization of the 
silver bromide and by lowering the concentration of the silver cations 
which lowers the velocity with which the reaction proceeds to the 
saturation point. Hence the precipitation phase is delayed, because 
of the delay in reaching a saturated solution of silver within the cell. 
The influence of a soluble bromide is felt chiefly in the earlier stages of 
development.* 

The Function of the Alkali. —According to the theory of develop- 
ment outlined in the preceding pages the reducing agents used for 
photographic development are considered as pseudo-acids having very 
small ionization constants but forming strongly dissociated salts. The 
function of the alkali is to assist in the ionization of the reducing agent 
and produce ionized salts, as in the case of hydrochinon 


OH NaOH (ye. INQ 
ae 
+ = + 2H,.0O. 


OH NaOH Ora ia 


Tue PuHysIcAL CHEMISTRY OF THE DEVELOPING PROCESS 


The Induction Period.—Even with the most energetic developers a 
short space of time elapses between the application of the developing 
solution and the first appearance of the image. This period is termed 
the induction period. The causes which produce this period are in 
general two: (1) the time required for the developer to penetrate the 


3 Zeit. wiss. Phot., 1904, II, 7. 
4See Arch. wiss. Phot., 1900, II, 76. Eder’s Jahrbuch, 1904, 1 


272 PHOTOGRAPHY 


film, including both the macro and micro phases of diffusion referred 
to in a previous section, and (2) the time required to saturate the solu- 
tion with silver in order that silver may be deposited and form a visible 
image. The actual duration of the induction period is controlled by 
the nature of the developing agent, metol and other energetic agents 
having a shorter period of induction than the lower energy developers 
as hydrochinon and glycin, the concentration of the developing solu- 
tion, temperature and the presence of a soluble bromide. A soluble 
bromide such as potassium bromide materially increases the duration 
of the induction period, particularly with developers of low energy. 
Soluble iodides on the other hand have an accelerating effect and 
shorten the period of induction.® 

The well-known Watkins method of factorial development is based 
upon the induction period. Watkins’ principle is that for any develop- 
ing agent the time required to produce the visible image is an accurate 
indication of the speed of development and is a certain definite frac- 
tion of the time necessary to reach any given stage of contrast. Any 
variation in concentration or temperature, etc., which would affect the 
time of development necessary to reach a given degree of contrast 
affects the time of appearance proportionately. In other words 


Ta = WT, 


where T is the time for density D, T, the time of appearance and W 
a constant depending on the developer. 
This statement is sufficiently near the truth to be of practical ap- 


plication but both theory and experiment show that this simple relation . 
does not hold exactly. The Watkins method of factorial develop- — 


ment will be referred to again in the chapter on The Technique of De- 
velopment. 

The Velocity of Development.—After the induction period is 
passed the growth of the image may be rapid or slow according to the 
conditions under which the process takes place. The principal factors 
which determine the rapidity of development are the same as those 
which influence the period of induction. 

A knowledge of the velocity of development is essential to the calcu- 
lation of the time required to reach a given stage of contrast (7) and 
is most conveniently and accurately determined by the method of 
Nietz.° It has been shown in the chapter on sensitometry that a 

5 For a full explanation of this interesting reaction see Sheppard and Meyer, 
Phot. J., 1920, 60, 12. 

§ Theory of Development, p. 80, 


THE THEORY OF DEVELOPMENT 273 


series of plates exposed under identical conditions in a sensitometer 
and developed for varying times from f, to t, produce a series of 
H. and D. curves the straight line portions of which meet in a point 


Std Log E Log E 
Fic. 171. Growth of Density with Time of Development. (Nietz) 


(Fig. 171). If we take any fixed exposure on the log exposure base 
and erect a perpendicular line we have the information desired, i.e. 
the growth of density with development since the time of exposure for 
each of the densities is constant. By plotting D,, D,, etc., as a func- 
tion of the time we get a curve of the exponential type (Fig. 172) 


7 
Dev. (Min.) 


Fic. 172. Curve Showing Growth of Density with Time of Development 
(Nietz) 


which shows that density increases rapidly at first and then less and 
less rapidly as development proceeds until finally a point is reached 
where development apparently stops and there is no further increase 
either in density or contrast. This, it will be readily seen, is in agree- 
ment with conditions observed in everyday practice. 


274 PHOTOGRAPHY 


The explanation of this progressive diminution in the velocity at 


which density increases is quite simple, although it is a difficult matter 


to find a mathematical expression which will cover all conditions. 
After development for any length of time short of that required to 
produce the maximum density, we have three kinds of grains present: 


A. Developed grains. ° 
B. Developable but undeveloped grains. 
C. Undevelopable grains. 


The A grains thus represent the density already attained; the A and 
B grains together the maximum density which can be secured exclusive 
of fog. The B grains, therefore, are the only ones subject to develop- 
ment and as the reaction proceeds the number of B grains will become 
less and less until finally when all are developed the process must stop. 
Thus the density undeveloped at any time ¢ will be (D,.— D), where 
D is the density developed at any time ¢ and D,, the maximum den- 
sity. Supposing that the rate at which the developer reduces the ex- 
posed but undeveloped grains is a constant and independent of the 
number of grains (as is actually the case) and that the rate of diffusion 
remains unaltered, we can express the rate of development or dD/dt as 

dD 

as k(D. — D), 
where F& is a constant determined by the rate at which the exposed 
grains are reduced by the developing agent. This formula fits the 
case fairly well with acid developers over a moderate range but wide 


variations are observed with most alkaline developers and other more 


. complex equations have been suggested to account for the various fac- 
tors involved. A comprehensive review of later work on the velocity 
of development and development velocity equations will be found in 
The Theory of Development by A. H. Nietz. 


The Velocity Constant—Now while the number of undeveloped — 


grains constantly grows less and less as development proceeds, the rate 
at which the grains are attacked by the developing agent remains con- 
stant. Thus if we have a total of 100 developable grains present at 
the beginning of development and at the end of the first minute of de- 
velopment one half of this number or 50 have been reduced to the 
metallic state, then at the end of the second minute of development the 


developing agent will have reduced to metallic silver one half of the 


grains which remain or 25, and so on as the time of development is 


. 
3 
. 
: 
; 
{ 
| 
‘ 
: 


THE THEORY OF DEVELOPMENT 2795 


prolonged. In other words, the developing agent reduces to the 
metallic state a definite proportion of the remaining developable grains 
for each unit of time which it is allowed to act. This proportion is 
termed the velocity constant of development. It is usually denoted 
by k. 

The velocity constant at the same temperature and with the same 
emulsion varies with the developer. It is different with different 
plates, being influenced by the conditions prevailing during the manu- 
facture of the plates. 

To determine the value of the velocity constant, k, we require to 
know the values of gamma for two sensitometric strips simultaneously 
exposed and developed for different times, of which one is double the 
other. The values of y, and y, having been found, k may be calculated 
from the following equation: 7 


The calculations are rendered simpler by the use of the following 
’ table worked out by Mees and Sheppard. To use this table divide 
y2 by y, and against the value of this dvidend in the table is the 
value of k for 5 minutes development. The value of k for any other 
time of development may be found by dividing 5 by the number of 
minutes development and multiplying by the value of k for 5 minutes. 
Thus if in a certain case the value for k is given in the tables as .215, k 
for 2 minutes development will be 


or X .215 = .538. 


Calculation of the Time of Development for a Given Gamma.— 
We are now in a position to calculate the time of development required 
to obtain a given gamma with any particular developer. While in all 
sensitometric work it is desirable that plates be developed to a gamma 
equal to unity, in practical work it is often desirable, owing to the re- 
quirements of different printing mediums, to develop to lower or even 
higher values of gamma than unity. Thus negatives to be printed on 
carbon or platinum require to be developed to a higher gamma than 
those destined for use with developing-out papers. Then again it is 
usually desirable to develop different subjects to different gammas and 
consequently it is an advantage to be able to calculate the time of de- 


7 Mees and Sheppard, Phot. J., 1903, 43, 48; Phot. J., 1904, 44, 297. 


276 


velopment to reach any gamma which may be desired. This is a com- 
paratively simple matter if we have determined the gammas of two 
sensitometric strips simultaneously exposed and developed for different 
times so that one is double the other. 
determined, the time of development required to reach any other 
gamma may be found either by the graphical method of Hurter and 


PHOTOGRAPHY 


These constants having been 


Driffield ® or that of Mees and Sheppard.® 


0.005 
0.010 
0.015 
0.020 
0.025 
0.030 
0.035 
0.040 
0.045 
0.050 
0.055 
0.060 
0.065 
0.070 
0.075 
0.080 
0.085 
0.090 
0.095 
0.100 
0.105 
0.110 
0.115 
0.120 
0.125 
0.130 
0.135 
0.140 
0.145 
0.150 
0.155 
0.160 
0.165 
0.170 
0.175 
0.180 
0.185 
0.190 
0.195 
0.200 


8 Hurter and Driffield, On the Control of the Development Factor. Phot. J., 


1903, 43, 16. 


0.205 
0.210 
0.215 
0.220 
0.225 
0.230 
0.235 
0.240 
0.245 
0.250 
0.255 
0.260 
0.265 
0.270 
0.275 
0.280 
0.285 
0.290 
0.295 
0.300 
0.305 
0.310 
0.315 
0.320 
0.325 
0.330 
0.335 
0.340 
0.345 
0.350 
0.355 
0.360 
0.365 
0.370 
0.375 
0.380 
0.385 
0.390 
0.395 
0.400 


9 Mees and Sheppard, Phot. J., 1903, 43, 48, 199. 


— 
. . 


be A sc ce ee ce oe ee ee oe oe 
Pe ae 


NNHHN YN 
NNW&® 
mT ODO U1 


.210 
.205 
.200 
-195 
.1QI 
.186 
.182 
.178 
-174 
.169 
.165 
161 
-157 
-154 
.150 
-147 
-143 


.139 
.136 


.216 7 


0.0010 
0.0008 
0.0008 
0.0008 
0.0006 
0.0008 


0.0006 © 


0.0008 
0.0008 
0.0006 


; 
4 
} 

. 

7 
a 


THE THEORY OF DEVELOPMENT 277 


The graphical method of Hurter and Driffield can be most simply 
explained by an example. Suppose y, to be 0.82 andy, to be 1.36. 
Take an ordinary H. and D. chart, such as used for plotting the char- 
acteristic curve, and call the base line divisions “ Minutes of Develop- 
ments’ and the ordinates ‘‘Gammas’’: then there are three points 


reds O32 O6825 125. 25 s 'o 20 an 30 “62 320 ©46 


is 2 15 
LANDSCAPE 13 ; 


ARCHITECTURE. § 
PORTRAIT .6 | 


: 0 ae tae 70 100 200 500 1000 
i@) 1 Ce le a) '6 7 
MINUTES ‘DEVELOPED=28 375 575 


m eererere 
Or 2 #345 7 13 af 3 4 8.7188 


Fic. 173. H. and D. Method for Calculating the Time of Development 
for Given Gamma 


through which a curve may be drawn—o, 0.82 and 1.36. Suppose y, 
(0.82) to have been produced by three minutes development and y, 
(1.36) with six minutes; then y, (0.82) is plotted on the 3 minute 
line and y, on the 6 minute line. A curve is then drawn through these 
points and zero. Then the time of development for any desired 
gamma may be obtained by drawing a horizontal line from the left- 
hand scale until it cuts the curve and dropping a perpendicular from 
the point of intersection to the base. In the example shown the times 
required to reach gammas of 0.80, 1 and 1.30 are found to be 2. 80, 
3.75 and 5.75 minutes respectively. 

For the second method as developed by Mees at Sheppard the 
values of gamma infinity (yo) and the velocity constant of develop- 
ment (k) require to be known. Methods of calculating these con- 
stants have already been given: for the former in page 252 and the 
latter in page 275. 

The values of these constants having been calculated for the case in 
hand, the time of development for any desired gamma may be ob- 
tained from the equation 


A ace Yo(L eh!) 


278 - PHOTOGRAPHY 


The actual calculations are rendered quite simple by the use of the 
tables (page 279) worked out by Drs. Mees and Sheppard for values 
of (J —e*t) and corresponding values of kt.*° 

From the above 

Yt = 
bee UF ene 
Yo ( z ) 
or 
Gamma required _ 


bensicccstentcolle Sid buckets NG ge 
Gamma infinity Cee 


Therefore to obtain the time of development for a given gamma, 
divide the required gamma by the gamma infinity of the plate. In the 
second column of the tables find the nearest lower value of (J — e~**) 
corresponding to the dividend. Opposite this in the first column of 
the tables will be found the value of kt corresponding to that obtained 
for (I— et). Divide the value of kt as given by the value of R, as 
previously found by calculation, and the result is the time of develop- 
ment required to attain the desired gamma. 

For example: 


Gamma infinity ==T1.6, 
Velocity constant == .I5, . 
Gamma required 08. 
Then 
0.8 aah ne Bees: 
By tases Bs (I — e~**), 


The nearest value of (J —e"*) in the tables which corresponds to .5 
is .5034, corresponding to a kt of .7oo. Dividing this by the velocity 


constant (k) .15 we obtain 4.7 minutes, or 4 minutes and 42 seconds, 


which is the time of development for a gamma of 0.80. 

Effect of Temperature on Development.—In common with nearly 
all chemical reactions, the rate of development is considerably in- 
fluenced by temperature. The effect of temperature on the time of 
development was first studied quantitatively by Houdaille in 1903 * 
whose work was followed up with a more complete investigation by 
Ferguson and Howard, Alfred Watkins and Mees and Sheppard.” 

10 Phot. J., 1904, 54, 207-8. 

11 Bull. Soc. Franc. Phot., 1903, 19, 256. 


12 Ferguson and Howard, Phot. J., 1905, 45, 118. Ferguson, Phot. J., 1906, 
46, 182. Mees and Sheppard, Investigations. Sheppard, J. Chem. Soc. (Lon- 


don), March, 1906. Ferguson, Phot. J., 1910, 50, 412. Mees, Phot. J., 1910, 


50, 410. Watkins, Phot. J., 1910, 50, 411. Watkins, Phot. J., 1909, 49, 367. 


. 


a 


a EEE 


THE THEORY OF DEVELOPMENT 


2 


lx 


( 


9 


TABLE OF CORRESPONDING VALUES OF ki AND I — e~*' FOR DETERMINATION OF 
TIME OF DEVELOPMENT FOR REQUIRED GAMMA OF PLATE OF GIVEN k AND y® 


(Mees and Sheppard, Photographic Journal, November, 1904, page 297) 


kt 


.000 
025 
.050 
075 
.100 
(725 
150 
175 
.200 
225 
250 
275 
.300 
325 
350 
375 
.400 
425 
.450 
475 


.500 © 


525 
-550 
575 
.600 
.625 
.650 
675 
.700 
725 
-750 
775 
.800 
825 
.850 
-875 
.g00 
925 
.950 
975 
1.000 
1.025 
1.050 
1.075 
1.100 
1.125 
1.150 
1.175 


diff. for 
.O1 kt 


.0095 


.0086 


.0077 


.0071 


.0064. 


.0057 


.0052 


.0047 


.0042 


.0038 


.0034 


.0032 


kt 


1.200 
1.225 
1.250 
1.275 
1.300 
1.325 
1.350 
1.375 
1.400 
1.425 
1.450 
1.475 
1.500 
1.525 
1.550 
1.575 


' 1.600 


1.625 
1.650 
1.675 
1.700 
1.725 
1.750 
1.775 
1.800 
1.825 
1.850 
1.875 
1.900 
1.925 
1.950 
1.975 
2.000 
2.025 
2.050 
2.075 
2.100 
2.125 
2.150 
2.175 
2.200 
2.225 
2.250 
2.275 
2.300 
2.325 
2.350 
2.375 


7 — (eke 


.6988 
-7959 
He oh 
-7203 
7275 
-7339 
-7493 
-7469 
7534 
-7592 
.7651 
-7710 
-7769 
.7822 
-7875 
-7928 
-7981 
.8029 


.8077 
8125 


8173 
.8215 
.8259 
.8303 
-8345 
.8387 
.8426 
.8465 
.8504 
8539 


8575 
S61! 


.8647 
.8680 
.8712 
8744 
.8776 
.8805 
.8834 
.8863 
.8892 
.8919 
8945 
.8971 
.8997 
9021 
9045 
.9069 


diff. for 
.OT Rt 


.0029 


.0024 


-0022 


-002I 


.OOI7 


.OO16 


0014 


0013 


-OOI2 


.OOT05 


280 PHOTOGRAPHY 


TABLE OF CORRESPONDING VALUES OF kt AND I — e~*! FoR DETERMINATION OF 
TIME OF DEVELOPMENT FOR REQUIRED GAMMA OF PLATE OF GIVEN k AND y® 
(Mees and Sheppard, Photographic Journal, November, 1904, page 297) 


(Continuation of page 279) 


diff. for ae - 
kt I — e-kt .o1 Rt kt 7 vaneake 
2.400 9093 } 3.200 9592 
2.425 .QII3 3-225 -QOOlI 
2.450 9135 -00086 3.250 .g61r { ‘20039 
2.475 .9157 3.275 .9621 
2.500 .9179 ) 3.300 9631 
2.525  “9t07 | oheys 3-325 9639 | 49036 
2.550  .9217 | 3.350 9648 
2.575 9237 3-375 ey 
2.600 .9257 3-400 -966 
2.625 9274 3-425 = -9u/4 
2.650 .9292 eute 3 3-450 .9682 noe 
2.675  .93I0 ; 3-475 .9690 
2.700  .9328 3.500  .g698 
2.725 9344 00064 ° 3-525 ae .00029 
2.750 .9360 ; 3-550 9713 
2.775 9376 3-575 9720 
2.800 .9392 3-600. .9727 
2.825 9408 00058 3-625 9732 .00026 
2.850 9412 ; 3.650 9739 
2.875 9426 3-675 9746 
2.900 .9450 3-700 9753 
2.925 .9463 .00052 3-725 9758 .00023 
2.950 .9476 ; 3-750 9764 
2.975 .9489 3-775 -9770 
3.000 .9502 3.800 9776 
3.050. .9525 3.850  —.9786 
3-075 9537 3-875. 9792 
3.100 9549 3.900  .9798 
3-125 9559 |} oo043 3.950 9807 | goor19 
3-150  .9570 3-975  .9812 
3.175 .9581 4.000 .9817 


The ratio of the velocity constant, k, for any two temperatures is a 
measure of the effect of temperature on the velocity of development 
within this particular range and for that particular developing agent 
and is termed the temperature coefficient of development (T.C.). 
The range of temperature chosen in practice is 10° C. (18° F.) so that 
the expression for the temperature coefficient becomes 


Rt C./ Re 10” 4, 


The temperature coefficients of a few of the more common developing 
agents are as follows: 


id able 


THE THEORY OF DEVELOPMENT 281 


PMeNCRMPPELIIOUC DIOMNde, 0. fdas cia reve ad Cee eluvc see ees 1.5 
PTO COINOE, i. isa uv a ev tle cave eh es Sev ewes est 1.9 
eR MRN POT OCRTINAL CLG, ccs sas ccs ed ya warns Ale a so hin ewe ees 1.9 
RE eIIIMPC TMS OL OMIC) oo au a bee vg) v0 os 9,050 vo sega canes 1.9 
ts, hc 5 fo siereelhs ie ee law ie eee ure ches Pen yart 2.3 
eee y ol ag gis.o fis. eck and win binleele) Sk gw wee wee ceed 2.2 
ee oa.) td a AD Ee eR GA picesch g.4 x Pacals bye. wogelg 8 b's 2.25-2.4 


As a general rule the temperature coefficient appears to be a char- 
acteristic of the developing agent, being for the most part unaltered 
by changes in the proportion of alkali to the developing agent, or by 
dilution, but it is much higher when bromide is used. 

Mees and Sheppard have shown that there is also a variation in 
the temperature coefficient with different plates, so that a calculated 
T.C. for a given developer will not necessarily hold if a change is 
made to another brand of plates. The temperature coefficient is ap- 
proximately constant, however, for different batches of the same 
plate. 

With certain developing agents of low energy, such as hydrochinon, 
low temperature not only slows development but has an action similar 
to that of a soluble bromide at normal temperature, i.e. the inertia is 
lowered and the lower tones retarded. 

Calculating the Temperature Coefficient.—As we have already seen, 
the time of appearance of the image is an indication of the velocity 
of development, hence we may calculate the effect of temperature on 
the rate of development with a given developing agent from the dif- 
ference in the time of appearance of the image at two different tem- 
peratures. A plate is exposed and then divided into two pieces (or 
two identical exposures made). One of these is developed at any 
convenient temperature and the time of appearance noted. The other 
is developed at a temperature several degrees higher or lower; 10° C._ 
(18° F.) being a convenient difference. The time of appearance at 
this temperature is noted. 

We now have the time of appearance at two different temperatures 
and from this the temperature coefficient may be calculated by the 
following formula: 


(log T, — log ta) X 10 

T° —¢° 
In other words, the difference in the logarithms of the two times of 
appearance, multiplied by 10 and divided by the difference in degrees 


= log of T.C. for 10° C. 


282 PHOTOGRAPHY 


‘Centigrade of the two respective temperatures, is equal to the loga- 
rithm of the T.C.%* 
Thus if the times of appearance are 30 and 20 seconds at 17.5° C. 
(63° F.) and 25° C. (77° F.) respectively, we have 


log 30 = 1.4771 
log 20 = rela from log tables. 


Difference = .1761 
xX 10 10 
. = 1.761 
7.5 = .2348 


log of 1.72 = temperature coefficient. 


A very ingenious graphical method devised by Mr. Alfred Watkins 
is even simpler and avoids all calculations whatsoever. The starting 
point on which his method is based is the fact that the time of develop- 
ment required to produce an equal gamma increases in logarithmic 
proportion while the temperature increases arithmetically. The 
times of appearance having been found for two different tempera- 
tures, a slip of paper is laid on the log scale of Fig. 174 and the times 
of appearance laid off against the corresponding values of the log scale. 
Beneath the marks are placed the respective temperatures. This slip 
of paper is then laid on the fan-shaped diagram and adjusted so that 
the two marks cut the lines of the two temperatures, the edge of the 
paper falling along one of the horizonal lines. The point where the 
paper slip intersects the radial temperature lines is marked with the 
proper temperature coefficient.’ 

As a result of extensive research, Watkins gives the following T.C. 
for several common developing agents: 


Pyro-soda (Watkins thermo formula), no bromide............ 1.5 
Pyro-soda (Watkins thermo formula), with bromide......... 1.9 
Pyro-soda (Hurter and Driffield formula).................. 1.48 
Pyro-soda (Kodak - powders) ......... 2% «atm mane ee 1.9 
Pyro-soda (Ilford formula) ...........2 2205 aoc 2.04 
Rodinal (also azol, victol and certinal). << 7.0.0 ee ee £05. 
Metol-hydrochinone (Watkins thermo formula)............. 1.9 
Glycin 0.6. e pesos ne oe ama pone nase glp palatine manne een 2.3 
Hydrochinon, 6.0). 606 sicales s.00 « td boule: eos one im ea 1.4 -2.25 
(Sheppard ‘and Mees). find. ......5..%..4 she ee 2.20-2.80 
Ortol ssos sce pore 00 a0 eet duce a6 up ece Sbm call el neem 2.06 


13 Ferguson, Phot. J., 1910, 50, 414; see also Phot. J., 1906, 46, 182. 
14 For other methods see Ferguson and Howard, Phot. J., 1906, 46, 182. 


——o.. 7 


283 


THE THEORY OF DEVELOPMENT 


‘) ZL 2y} Buryepnosjed JO poyyyy suUDpeM ‘VLI “SI 
zIwIS) = DI WH 1tYv907 
( 


oricsosgL 09 0S OF OF o Si O16 3 


JTW 


Ly 50S £9 OL ers) 
A1v9S JyNLVYIdWIL 


; 
b 


; 
Time of Development at Various Temperatures.—The time of de- | 
velopment required to reach any given gamma and the T.C. for the | 
same plate and developer having been obtained, the time of develop- 
tnent at various temperatures is very easily found. Place the edge of 
a sheet of paper on the horizontal line corresponding to the T.Ce6is 
the developer and mark off the points of intersection with the tempera- 
ture lines. Transfer this paper to the log scale, p!acing opposite the 
time of development in minutes or minutes and a fraction, the cor- 
responding temperature at which the examination was made. This 
liaving been done the times of development at other temperatures 
necessary to reach the same gamma may be written down directly 
from the log scale.*® 

There is, however, no actual necessity for knowing the temperature 


284 PHOTOGRAPHY 


Time in minutes 


0° 


Temp. F. 
Fic. 175. Stokes Time Development Chart 


coefficient in order to determine the time of development for various 
temperatures. Since the time of development for a given gamma 
progresses logarithmically as the temperature progresses arithmeti- 
cally, if the time of development at two different temperatures is 
known, a straight line drawn through these two points when plotted 


15 This simple graphical method of drawing up a table for the time of de- 
velopment at various temperatures was first indicated by Mr. Alfred Watkins. 


THE THEORY OF DEVELOPMENT 285 


on a log scale of times of development as ordinates against an even 
division scale of temperatures as abscisse (Fig. 175) will indicate, for 
all practical purposes, the time of development at all intermediate 
points. This method is due to Mr. W. B. Stokes.'® 

The Action of Soluble Bromides in Development.—The customary 
addition of a certain amount of soluble bromide, which is nearly al- 
ways potassium bromide, to a developing solution for the purpose of 
preventing “fog” materially affects the normal course of develop- 
ment. 

For an unbromided developer the inertia is constant with increasing 
times of development, but this is not true in the case of a developer 
containing a soluble bromide in which case at the same degree of de- 
_ velopment there is a lateral shift of the curve to the right. This is 
illustrated in Fig. 176%" where the solid lines represent the curves of 
the unbromided developer for three different degrees of development 
and the dotted lines the curves of the bromided developer for similar 
degrees of development. It is evident that if the curves of the 
bromided developer are produced below the log E base they will meet 


Unbromided 
ereren Bromided 


Fic. 176. Effect of a Soluble Bromide in the Developing Solution on the 
Plate Curve , 


in a common point. As the concentration*of bromide is increased 
this point of intersection moves slowly downward as shown in Fig. 
177. The amount of the downward shift, termed the density depres- 
sion, produced with a given concentration of bromide, is dependent 
upon the developing agent, being in general greater with low energy 
developers as hydrochinon than with those of greater energy such as 
‘ paraminophenol and metol. 

16 Brit. J. Phot., 1921, 68, 97. 


17 Sheppard, Photography as a Scientific Implement, p. 151. 
20 


286 PHOTOGRAPHY 


Bromide is without effect on the velocity constant k,’* and investi- 
gation shows that its effect on the general velocity of development is 
felt chiefly during the earlier stages; the induction period and that 
immediately following. 

Perhaps an even more readily comprehensible method of presenting 
the action of a soluble bromide in development is that adopted by 
Watkins in the Watkins Manual. Fig. 178 represents a subject of 


Fic. 177. Density Depression with a Soluble Bromide. (Nietz) 


four gradations for a given degree of development in an unbromided 
developer. The lower illustration represents the same exposure de- 
veloped to the same stage in a bromided developer. It will be observed 
that, while the contrasts of both are equal, the action of bromide has 
reduced the tones considerably and this depression is more noticeable 
in the lower tones than the higher. In fact the addition of bromide 
has prevented the lowest tone from appearing at all. ‘The effect of 
bromide is to actually reduce the speed of the plate. As the time of 
development is increased and a higher gamma is reached, the lower 
tones will develop out, so that in order to restrain the development 
of the shadow detail in over éxposed plates development must be com- 
pleted before the bromide has lost its restraining action. The use of 
bromide for this purpose, however, falsifies the gradation ‘of the 
negative. _ 

Theoretically gamma infinity is unaffected by the reasonable addi- 
tion of bromide, but in practice, owing to the absence of fog, the print- 


ing contrast of a negative developed to the same gamma may be higher ~ 


for the bromided than for the unbromided developer. 


18 Nietz, Theory of Development, pp. 124, 170. 


THE THEORY OF DEVELOPMENT 287 


The restraining action of bromide is greater on fog than on the 
image, hence, even in cases of underexposure, a small amount of 
bromide may be advisable in order to prevent the appearance of fog 


a 
ma 
_ 


a= 


A B c D 
Fic. 178. Effect of Soluble Bromide on the Densities. (Watkins) 


due to development being forced beyond the usual limits in order to 
secure all possible shadow detail. 

The Relative Reducing Energy of Developing Agents.—The effect 
of a soluble bromide at the same concentration varies with the develop- 
ing agent but is constant and characteristic of that particular agent. 
Use was made of this property by Sheppard and Mees to compare 
different developing agents as to their relative reducing or developing 
energy known as the reduction potential. For a given concentration 
of bromide under fixed conditions the depression of density will be 
dependent upon the ability of the developer to overcome the resistance 
of the bromide. Developing agents of greater energy will require 
larger amounts of bromide to produce the same depression of density 
than those of lower energy; hence the concentration of bromide re- 
quired to produce a given density depression will be in direct pro- 
portion to the energy of the developing agent.’® 

Taking the bromide concentration required to produce a given de- 
pression of density as unity, Nietz obtained the following scale rep- 
resenting the relative energies of the more common developing agents: 


ee iy oe ba ove 6 die ie DA wane a eee bw ble bes 0.3 
p-phenylene diamine hydrochloride (no alkali).................. 0.3 
p-phenylene diamine hydrochloride (with alkali)............... 0.4 
ay eee MN oe occ isin u,v o eighties pt aa best awe ne ves 1.0 Standard 
ee PIV CMI ots ek nea eis tose stb eneensds tesevavens 1.6 
A ERRNO MIO ots cin Gs tee cee bes a's Sas ces pe sa ws SA ete eee 2.0 
OT a ea eg eee 2.2 


19 See Sheppard and Mees, Investigations, p. 188. Nietz, Theory of Develop- 
ment. 


eres 


288 PHOTOGRAPHY 


p-amidophenol (hydrochloride) ........iciss0.42-05589 0a ral Sie OO 
Chlorhydrochinon. (adurol) ........0:. «0.0 «se 206 = 9 sy apn ne 7.0 
Dimethyl p-aminophenol sulphate............+0.... see 10.0 
Monomethyl p-aminophenol sulphate (metol)..............00- . -20.0 
Diamidophenol’)(amiidol) ..2.:0. 54 28s) ek be > boas ie 30-40 


In general the higher the value of the reducing energy the higher 
is gamma infinity, but there are several exceptions which are not yet 
completely understood. Contrary to what might be expected, there 
appears to be no direct relation between the fogging power of a de- 
veloper and its reducing energy or reduction potential. 


GENERAL REFERENCE WorxKS 


Eper—Ausfuthrliches Handbuch Photographie, vol. 1V, 1905. 
Hust—Entwicklung ‘der Photographischen Bromsilbergelatineplatte, 1922. 
LuTHER—Die chemischen Vorgange in der Photographie, 1899. 
Nietz—Theory of Development, 1922. 
Reiss—Entwicklung der Photographischen Bromsilberge' ater ee 
SEYEWETzZ—Le Negatif en Photographie, 1922. 


i i a el 


CHAPLER. XII 
ORGANIC DEVELOPING AGENTS 


Developing Power.—The sensitive emulsion, as we have seen, con- 

sists of certain halide salts of silver in an extremely fine state of 
division held in a colloidal medium. We have already considered in 
the chapter on the latent image the various theories proposed to explain 
the nature of the change which occurs when the sensitive silver salts 
are exposed to light. While we do not know the composition of the 
latent image, we do know that there are certain chemical compounds 
which possess the property of reducing to metallic silver those grains 
of silver halide which have been affected by light. Such chemical sub- 
stances are known as developers since they “ develop,” or render visible, 
the latent image formed by light. All developing agents are reducers, 
but not all reducers are capable of photographic development by any 
means. We are not yet in a position to say definitely what constitutes 
developing power; i.e. what must be the chemical composition of a 
substance in order that it may function as a developer. The general 
conclusions of Lumiere and Andresen bearing on this subject will be 
discussed later. 
_ While in common speech a developer is taken to mean either the 
developing agent or the solution used for development, in this chapter 
we are concerned primarily with the developing agent and all reference 
to a developer applies to a particular agent such as metol, pyro, etc., 
and not to a developing solution as applied to the plate. This is al- 
ways termed the developing solution. 

Classification of Developing Agents.—A comparatively large num- 
ber of substances possess the property of developing exposed silver 
halide but for various reasons only a few of these are of practical 
value. Eder? divides all possible developing substances into three 
classes : 


1. Those which develop a definite part of the latent image before fog 
sets in. (Common developers. ) 


1 Ausftihrliches Handbuch der Photographie, p. 288 et seq. 
289 


290 PHOTOGRAPHY 


2. Those which develop energetically with a minimum of alkali but 
produce serious fog. (Powerful developers. ) 

3. Those which scarcely develop the latent image at all even with a 
maximum of alkali and yet develop fog vigorously. 


A somewhat more comprehensive classification is adopted by Nietz.? 


1. Developers having too low reducing energy to be useful practically, 
e.g. ferrous citrate. 

2. Developers giving undesirable reaction products in developing, e.g. 
hydrazine. 

3. Developers too powerful for ordinary use, e.g. triaminophesak 

4. Developers of practical utility, e.g. all ordinary developing agents, 
metol, pyro, paramidophenol, etc. 


Only this last class will be discussed in the present chapter although 
references are made to several of the others in the bibliography at the 
close of the chapter. 

The Source of Organic Developing Agents.—One of the most 
fertile fields of the research chemist in modern times has been that 
branch of organic chemistry which is concerned with the products re- 
sulting from the destructive distillation of coal. Among the long list 
of organic compounds which science has prepared from what was 
formerly considered to have little or no value are found practically all 
of our modern developing agents. 

Benzene, the father of the numerous aniline and phenol dyes, and 
likewise of our organic developing agents, was discovered by Faraday 
"in 1825, but it was not until 1866 that its structural formula was de- 
termined by Kekule. This consists of a hexagon with the carbon and 
hydrogen atoms linked together around the six points. 


CH 


CH 
(1) 
1. The structural formula of benzene after Kekule. 


2. Its abbreviated form generally referred to as the benzene nucleus. 
3. The points of substitution. 


2 Theory of Development, p. 14. 


—aV es eee el eee eee ——— = 7 


a 


. 
' 
i 
4 
: 


ORGANIC DEVELOPING AGENTS 291 


‘The atoms of hydrogen at any of the six points of substitution may 


be replaced by atoms of chlorine, hydroxyl or amido groups and, as a 
substance with entirely different properties is formed according to the 
group substituted and the point at which substitution is made, it can 
be readily seen that a very large number of compounds become pos- 
sible. By substituting chlorine, hydroxyl or amido groups in the first 
position we secure: 


Cl —NH, 
Chloro-benzene eS Bea aes Amido-benzene 
or Phenol or Aniline 


None of these compounds has any developing power. However, if 
we replace the two hydrogen atoms at positions 1 and 2, at I and 3,. 
or at. 1 and 4, we get three hydroxy-benzenes having the formula 
C,H,(OH), and identical in composition but differing in constitution. 


OH 
OH OH 
—OH vie 
| OH 
Ortho- Meta- Para- 
hydroxy-benzene hydroxy-benzene hydroxy-benzene 
or Pyrocatechin or Resorcin or Hydrochinon 


Substitution in the 1 and 2 positions is termed the ortho position, 1 and 
3 the meta position, and 1 and 4 is termed the para position. Of the 
three compounds two are developers, para-hydroxybenzene being the 
agent known as hydrochinon while ortho-hydroxybenzene is known 
commercially as pyrocatechin. The third compound, meta-hydroxy- 
benzene or resorcin, has little or no developing power. 

By replacing the hydrogen atom in the second position of para- 
hydroxybenzene, or hydrochinon, with chlorine, Hauff produced mono- 
chlor-hydrochinon (C,H,C1(OH).) which was introduced commer- 
cially as Adurol. Schering substituted bromine in the same way and 
obtained mono-bromo-hydrochinon (C,H;Br(OH),) which also was 
introduced as Adurol. 


292 PHOTOGRAPHY 


—OH —OH —OH 
6 3 Br 
—OH —OQH —QH © 
Hydrochinon Adurol (Hauff) Adurol (Schering) 


Having dealt with the developing agents formed by substituting two 
hydroxyl groups in the benzene nucleus, let us see the effect of adding 
a third. There are three positions also which we can obtain by this 
treatment: that in which all three groups are close together, or ad- 
jacent; that in which two are contiguous, and the third separated by 
one position; and lastly that in which the groups are symmetrically 
placed. 


—OH 3 —OH —OH 
—O04e —OH 
—OH onl —OH 

' OT 
Pyrogallol Phloroglucinol 1,2, 4 Trihydroxy- 
benzene 


These are known as adjacent, asymmetrical and symmetrical tri-hy- 
droxybenzenes or as pyrogallol, oxyhydrochinon and phloroglucinol. 
Pyro is the only one of these substances used as a developer. 

Precisely the same condition of affairs applies when the substitution 
is made with amido groups instead of hydroxyl groups. Thus we may 
have ortho, para, or meta amidophenols, or we may substitute instead 
two amido groups or one amido and one hydroxyl group thus produc- 
ing a whole series of amido-hydroxybenzenes. Pursuing the same 
idea further we may replace one of the hydrogen atoms with a methyl 
group (CH,). . 

For example if we introduce an amido group in place of a hydrogen 
atom in the fourth position in phenol we obtain para-amido-phenol 
which is well known as the base of such prepared developers as 
Rodinal, Azol, Activol, etc. 


aE OH 


NH. 


Phenol Paramidophenol 


ae ee a a en re 


q 


ORGANIC DEVELOPING AGENTS 293 


The introduction of a second amido group produces di-amido-phenol 


or the familiar amidol. 
OH 
: NH; 
2 NH, 


- —OH 
Paramidophenol Amidol 


There are three more developers formed from paramidophenol, metol, 
ortol and glycin. If paramidophenol be taken, and one hydrogen 
atom of the amido group be replaced by a methyl group we secure 
mono-methyl-paramidophenol. The sulphate of this is sold commer- 
cially as metol. 


u 3 
H. NH—CHs3. 
Paramidophenol Metol (base) 


Ortol is a mixture of hydrochinon and the sulphate of methyl-ortho- 
amidophenol. The probable formula is 


a \Ounia a 


Za 


Glycin is produced by inserting the carboxyl group in place of a 
hydrogen atom in the methyl group of metol, being para-oxyphenyl- 


glycine. 


NH.CH,COOH 
Glycin 


294 PHOTOGRAPHY 


There are two other developers derived from benzene, diphenal and 
paraphenylene diamine; thése, however, are not very important. The 
last is occasionally used for lantern slides and transparencies on ac- 
count of the very fine-grained images which it produces and ‘would be 
useful for line work were its contrast-giving properties greater. 


| NHe 
eee 
a NH; 
NH.HCl 
Diphenal 
(Andresen) Paraphenylene diamine 


If two benzene nuclei are joined together, as shown below, we ob- 
tain a body called naphthalene. If we introduce into this hydroxyl, 
amido and sulphonic acid groups we obtain a substance which may be 
termed 8 amido, 8, naphthol, @, sulphonic acid, known to photogra- 
phers as Eikonogen. 


JO Aa 


Naphthalene Eikonogen 


The relationship of the various developing agents and some of the 
methods of derivation are shown in the following family tree of the 
coal-tar developers as compiled by Dwight R. Furness.* All of the 
methods are not shown, only those of importance are dealt with for 
the sake of simplicity. 
The Significance of Group Relations.—Most of our knowledge of 
the structure of developing agents and the relation of the structure to 
developing properties is due to A. and L. Lumiere and to Andresen. 
The papers of these investigators (see bibliography at end of chapter) 
have established some general rules for the structure of compounds 


8 Phot. J. of Amer., 1918, p. 337. 


4 


ORGANIC DEVELOPING AGENTS 295 


which possess developing power. It is now generally accepted that 
the presence of hydroxyl or amido groups, either alone or in com- 
bination, is necessary in order that a substance may function as a de- 
veloper. 


PIT COAL 
LIGHT TAR OIL MEDIUM OIL HEAVY TAR OIL 
Many Products 
Benzene (Toluene, Xylene Phenol, Naphthalene, Lubricating 01! 


Sodium-«,amido 8, naphthol 2, sulphonate 

Eikonogen”) 
Sodium-amidonaphtholdisulphonate 
~ (Diogen”) 


Nitrobenzene Benzaldehyde Phenol Resorcin 


Trigmidoresorcin Diamidoresocin 


Phenyl-hydroxylamine Aniline 
(“Reducin”) 


Paramidophenol Quinone 
(“Rodinol "= Hydroquinone 
hydrochloride) eas 


Nitrophenol Dinitrophenol 
Seed 
Diamidophenol 


Paramidophenol Orthoamidophenol (“Amido!”) 


Methyl - 


_ pzoxyphenyl- | orthoamidopheno! 
glycocoll (+H lydroquinone = 
(“Glycin”) “Orto!”) 


Monomethyl- Resorcin Pyrocatechin Hydroquinone 
paramidophenol (‘Kachin’) 
« (“Scalo/”) p ! ‘ : 
me Pyrogallic Acid Monobrom Monochlor - 
pean ey eye) (Made from'ballicAcid) hydroquinone hydroquinone 
(“Adurol") 


With substances which contain in one benzene nucleus at least two 
hydroxyl groups, two amido groups, or one hydroxyl and one amido 
group: 


1. The substance is a developer only when the groups are in the 
ortho or para position. Meta compounds, so far as known, 
have no developing power. 

2. In general para compounds possess greater energy than do ortho 
compounds. 

3. The di-oxy-benzenes are more powerful than the amidophenols 
which are in turn more powerful than the diamido benzenes. 


296 PHOTOGRAPHY 


4. The developing power is not destroyed by additional Aside or 
amido groups. 

5. In the naphthalene series it is not necessary that both groups ie 
joined to the same benzene nucleus. The general rules re- 
garding developing function do not apply to this group. 

6. The substitution of chlorine or bromine for hydrogen increases the 

developing energy. 

. A substance containing two hydrowy groups requires an alkali, 

while substances containing two amido groups or one hydroxyl 
and one amido group do not require an alkali, 


=a 


In substances containing three hydroxyl or amido groups either 
alone or in combination: 


1. Symmetrical arrangements, as 1, 3, 5, have no developing power. 
Other arrangements differ in developing energy but no definite 
rule has been found to apply. 

2. Hydroxyl-phenols, containing three hydroxyl groups, can develop 
without alkali but are not practical when so used. 

3. Increasing the number of amido groups increases the energy of the 
developing agent. 


“Slow ”. and “ Rapid” Developing Agents.—It is necessary that 
we consider at this point the nature of the difference between the 


so-called “slow ” and “ rapid” developers. With a developing agent 


of the type represented by metol, amidol and the paramidophenol 
compounds, the time of appearance is very short, but the image sub- 
sequently builds up very slowly. With one of the so-called “ slow ” 
developing agents, such as hydrochinon or glycin, the time of appear- 
ance is much longer, but the image builds up rapidly. This difference 
in the two types of developing agents is shown in Fig. 179. 

Graded strips which had received the same exposure were de- 
veloped in metol and in hydrochinon side by side and a strip removed 
from each solution at regular intervals. After 1 minute all the steps 
were visible on the metol strip but only the heavier exposures on the 
hydrochinon strip. At the end of two minutes, the lower tones cor- 
responding to the shadows were just visible on the hydrochinon strip, 
but at the end of four minutes both strips looked almost alike. It is 
evident, therefore, that in the first case the image has appeared in full 
detail at an early stage, while contrast has only been built up with pro- 
longed development. In the second case, the image appeared com- 


4 

| 
: 

q 
9 

4 


; 
| 
q 


ORGANIC DEVELOPING AGENTS 297 


paratively late in development while contrast built up steadily from 
the beginning. Therefore when the progress of development is 
judged by inspection, the tendency is to remove the plate from the de- 


fou bd 
2MIN. 2MIN. 4min. 4min. 


Fic. 179. Slow and Rapid Developers. (Crabtree) 


veloping solution too soon when using developers of the first class and 
too late when using developing agents of the latter class. Owing to 
this fact the former are frequently referred to as soft-working de- 
velopers and the latter as hard-working, although this is not strictly 
correct.: : 

In Explanation.—The following pages are devoted to the character- 
istics of the modern organic developing agents. Each of the more 
important of these has been treated in the following outline: 


Chemical name, 

Appearance and solubilities, 
Characteristics as a developer, 
Representative formulas. 


The developers have been classified in alphabetical order by trade 
names. Several developers no longer on the market, but once popu- 
lar, have received brief treatment. 


Fe ae ay adele “a 
i é 

x as . 

*] 2 ae 


oe 
* 


298 PHOTOGRAPHY 


-¥ 


Adurol.—Mono-chlor-hydrochinon, C,H,(OH).Cl; mono-bromo- 
hydrochinon, C,H,(OH),Br: 


OH | OH 
5) Cl | Br 
OH we: OH 


In 1898 Hauff, and independently Luppo-Cramer, investigated the 4 
possibilities of securing a better developer by modifying hydrochinon 
by the substitution of bromine, chlorine or other halogens for one of 
the hydrogen atoms in the hydrochinon nucleus. These investigations 
resulted in a developer introduced by both Hauff and Schering as 
Adurol. The Adurol of Hauff is mono-chlor-hydrochinon while the 
mono-brom product is made by Schering. Both are alike in general 
properties although Lumiére in 1914 found that the mono-brom 
product is the more energetic of the two. 

It is a white crystalline powder which dissolves readily in water. 

As compared with hydrochinon, Adurol keeps better in solution, is 
not so sensitive to temperature, gives density more readily, even when 
used with alkaline carbonates instead of caustic alkalis and is more 
energetic in action; standing in this respect about midway between 
hydrochinon and the rapid soft-working developers such as metol and 
paramidophenol. The color of the deposit is blue-black and very 
suitable for lantern slides or transparencies. 

A formula follows: 


| 
: 


T. Adurol sic Vien. « otaidataca tag Rie re 85 gr. 19.5 gm. 
Sodium, sulphite -Cdry)s4.ci onic eee ee 382.3 gr. 87.5 gm. 
Water to make... 25.5 accusers eee 10 08; 1000 cc. 

Il.. Potassium. carbonates: .s2-) 6.5 1% oz 125. gm 
Water to makes 4 Wace 3y ee ee 10 Oz 1000 cc 


For studio and instantaneous exposures take equal parts. For full 
exposures outdoors take: : 


Solution. Ii... ce v cee selene oc) esas «nine orten ne I part 
Solution IID... ns.¢sabeees one ne eho sey Sten I part 
Water ....cccceccctcugaet ss piltiely ive Mee inti I part 


ORGANIC DEVELOPING AGENTS 299 


For a concentrated single solution the following is recommended : 


PE TLV ) onc ec bce nacre kecsccescsees 2 OZ. 200 gm. 
PMI CATODOAIC Sele. cate ca vice cevecsvocsecs s OF. 300 gm. 
Oe UTS SR ee a IO Oz. 1000 cc. 
When completely dissolved add: 

SM CGT tie vs ce sieve kav ccects Y oz. 50 gm. 


For studio and instantaneous exposures dilute with three parts of 
water. For full exposures take one part to five parts of water. 
Amidol.—Diamidophenol hydrochloride, C,H,(OH) (NH,),2HCI: 


OH 


NH. 
ree 


NH, 


Amidol was introduced by Hauff in 1892 and is now made by a 
number of firms. Dianol (Lumiére), Acrol (Eastman), Nerol 
(Schering) and Amidol-Johnsons are all products practically identical 
in composition. The commercial product takes the form of steel-blue, 
needle-like crystals which remain clear and colorless in solution, the © 
solution loosing its developing energy without the visible coloration 
which accompanies the oxidation of other developing agents. One 
part of amidol is soluble in four parts of water and the solubility in- 
creases rapidly as the temperature is raised. 

Amidol differs from most other developers in common use in that 
it develops without alkali. It may also be used in an acid solution. 
It belongs to the class of rapid working developers, the image ap- 
pearing very quickly with full detail and density growing slowly with 
time. It is rather sensitive to temperature and the working solutions 
should be kept as near as possible to 65° F. It is comparatively in- 
sensitive to potassium bromide, small amounts of which act only as 
a clearer, while quite large amounts are required to restrain develop- 
ment. Amidol is recognized as the finest developer for bromide 
papers owing to its rich velvet-blue-black and black tones and its free- 
dom from fog or stain. It is also ideally suited to the development 
of lantern slides and transparencies, for which purpose it is excelled 
only by ferrous oxalate. 


300 PHOTOGRAPHY 


The following are excellent formulas. Owing to the rapid loss of 
developing energy in solution it is advisable to add the dry amidol 
immediately before use, care being taken to dissolve the same thor- 
oughly in order to prevent spots. On no account can the solution be 
kept over a day. 


Wrater: 2.07 5. 5G wan be ee eee a 20 02. 500 —s cc. 
Sodium stlphite (dry) o..ra.dees5 2 ee ee 325 gr. Io gm. 
AMAIMOL Cos sins vee bos oe eeioea yee ee ee 50 er. a) 
Potassiam. bromides... i550 tenes eee 10 gr. 75 gm. 


Absolutely pure sulphite should be used in preparing the above. 
A sodium sulphite solution to which potassium metabisulphite has 
been added will keep in good condition for several days and by adding 


amidol a fresh developer can quickly be made as required. ‘The fol-— 


lowing is the B. J. Almanac formula for the neutralized sulphite solu- 
tion : 


Sodium sulphite: (dry)i¢...0. 4... 2 be ee 2 02: 100 gm. 
Potassium metabisulphite. . 2... .s.esaeg ee 1% oz. 25. gm. 
Water to make., ...swss écieeus Jao s eee ee re a 20 oz. 1000 cc. 


After the chemicals have been dissolved it is well that the solution be 
boiled. Boiling is not essential but it improves the keeping quality 
of the solution. The developer is prepared as follows: 


Amnidol is) 04 «5 aisinisit ne die iety <b mecca ee 40 gr 4.5 gm. 
Stock’ solution. >. . occc ae ys eos ten «Gna 4 Oz 200 cc 
Water” i.e. cen Coie go ates ay 20 02. 1000 cc. 


Preservatives of Amidol Solutions——A number of methods have 
been suggested for preserving solutions of amidol so that stock solu- 
tions may be prepared for future use. 

Namias has advised the use of 25 grains (.05 gm. per 1000 cc.) of 
boric acid per ounce of solution as an effective preservative and ac- 
cording to Crowther glycollic acid in proportion to 1/10 the quantity 
of sodium sulphite is even more efficient.* According to Namias a de- 
veloper of the following composition retains its activity for a long 
time owing to the preservative action of metol:® 


Amidol 22. <2... +sauculs seiner sie se sae 5 gm. a. er, 
Metol ....... s «¢ ve a) Gas dele eaters ne Seal I gm. 4.5 gr. 
Sodium sulphite (dry) .. acc. - a) es ee 20 gm. 87.5 gr. 
Potassium bromide... 7.70.24 a0) evens eee 2 gm. 88 gr. 
Water to make: . is occ Aa eee ".. 1000 cc. I0~ oz. 


4 Brit. J. Phot., 1920, 67, 642. 
5 Prog. Foto., 1921, p. 45. 


—— ee ee 


ORGANIC DEVELOPING AGENTS 301 


Bunel has advised the use of lactic acid as a preservative of amidol. 
To each 1000 cc. of the following amidol developer are added 5 cc. 
(1:50) of lactic acid (sp. gr. 1.21) :° 


MA le pan veccaccesececcsssee 5 gm. 229%, 
Pe IIBSM SULT TICE. HCUTY os ois F cs leecle Govan secdus 30 gm. 131 gr. 
A PRMITIR EU NG See Ghirs aici eds ga es Hales dale ga eu 1000 cc. IO Oz. 


M. J. Desalme has recommended the use of stannous (tin) chloride 
as a preservative of amidol. About one part of the following stock 
solution is added to each 25 parts of developer: 7 


CCITT IO COV SIA Le. os ves ns vey ma vee vne se 5 gm. T.6..-oz, 
RIUM PEC OW UCL foe 5 cst ob eae cons cena ven. 7 gm. 2.03 Oz. 
RNS et Wee yl cee wx o's sch ees cedte's 30 Cc. IO Oz. 


After cooling slowly to nearly room temperature the above mixture 
is poured slowly and with constant stirring into a cold solution of 


PO ACNE (ULV) ole. sista cee cose II gm. I 0z., 44 gr. 
RNP ec cin vic eds Geevcu wavvede 60 cc. 6 oz. 


The whole is then made up to a total bulk of 100 cc. (10 0z.), al- 
lowed to stand for twelve hours and filtered. 

In using amidol, which is not suitable to the employment of an 
alkaline salt, the stannous tartrate solution is first neutralized, or 
rendered slightly acid, by addition of sodium bisulphite, litmus paper 
being used to determine the point at which a slightly acid condition is 
reached.” 

Amidol prepared from 2: 4 dinitrophenol by reduction with tin and 
hydrochloric acid retains small quantities of tin chloride and thus when 
made up with sodium sulphite forms a solution which keeps well. 
The observation has been patented, No. 2070 of 1922.8 

Certinal—This is merely a trade name for a paramidophenol de- 
veloper similar to Rodinal. It is marketed by Ilford Limited, London. 

Edinol—_C,H,OHCH,OHNH,, sulphate of oxymethylparamido- 
phenol: 


OH 
CH,OH HSO, 
ot ssp aba tne 
2 
NHe 


6 Bull. Soc. franc. Phot., 1921, p. 290. 
7 Bull. Soc. franc. Phot., 1921, p. 192; Brit. J. Phot., 1921, 68, 359. 
8 Brit. J. Phot., 1922, 69, 81. 

21 


302 PHOTOGRAPHY 


Edinol was introduced as a developer by Bayer in 1901. It comes 
in yellowish crystals which are very soluble in water. The solubility in 
plain water is 15.9 grams per liter and in a five per cent solution of 
sodium sulphite plus an equal amount of sodium carbonate the solubil- 
ity is 9.7 grams per liter. It is free from tendency to fog and is non- 
staining. Neither does it affect the skin as do some of the other or- 
ganic developing agents, notably metol. It is not very sensitive to bro- 
mide and may be used alone or in combination with hydrochinon or 
adurol. As a developer it stands midway between the rapid soft- 
working developers such as metol and paramidophenol and the slow 
contrast developers as hydrochinon. It is particularly suited to the 
development of lantern slides and transparencies and in combination 
with hydrochinon forms an excellent developer for bromide and gas- 
light papers. 

For soft portrait negatives: 


Sodium. sulphite. (dry) ...00 4% 555.4 suse = 2 0z 100 gm 
Eedanol 20, icds «gine os ce toh cele eee 100 gr. II gm. 
Sodium carbonate (dry)c..c stesso ae ee tot 50 gm. 
Water to make... dis baasunn's see ee 20 Oz. 1000 cc. 


For contrasty negatives and general outdoor work: 


Acetone sulphite. os sc..s eeu acee peo ee 288 gr. 99. Po, 
Sodium sulphite (dry). ... 6.25 <ss 0s see ee 14 oz. 75 gm. 
Exxliniol oc. oi ee ae eae oe 100 gr. II gm. 
Potassium carbonate. ........:c. 40s. eee 2 oz. 100 gm. 
Potassium bromides:. /.. 2: - sss bee 50 gr. 5.5 gm. 
Water:.to make.ccisics dee abs eee ee 20 02. 1000 «CC. 


Eikonogen.—Sodium-amido-8-naphthol-B,-sulphonic acid, C,,H,- 
OHNELSO Na 
NH, 


“OH 
SO;H— J 


The developing powers of Eikonogen were observed by Meldola 
but the substance was not introduced commercially until Andresen 
placed the same on the market in 1889. The compound is supplied 
commercially as semi-opaque, yellowish crystals, sparingly soluble in 
cold water but more easily in hot. The solubility at 15° C. (59° F.) 
is given as 7.6 grams per liter while the solubility in a five per cent 


tah 
eo 


ORGANIC DEVELOPING AGENTS 303 


solution of sodium sulphite is 8.2 grams per liter at the same tempera- 
ture. The substance is very susceptible to oxygen and should be kept 
closely corked to prevent oxidation. The commercial product is said 
to contain small quantities of potassium metabisulphite to prevent oxi- 
dation. 

As a developer Eikonogen is rapid, though not so energetic as metol 
or amidol, and tends to give soft detailed negatives. It is very suitable 
for work in which soft negatives are required as in portraiture, but for 
most purposes it is combined with hydrochinon in order to obtain suffi- 
cient contrast. It is rather sensitive to potassium bromide which must 
be added with caution, and is non-staining and non-fogging. 

The following formula for Eikonogen alone is excellent for soft 
negatives : 


Mer eeHINIte (COPY) cos cs apiece nes ce ceees eens 50 gm. 
Ny 64 ain sks Soa we dus ov nde eees 1% Oz. 25 gin, 
meerlemowater tO Make...) ek ee a ee 20 Oz. 1000 Cc. 

WeetrCeeR iy CAPHONAtC. <<. ese ce ee i eee ws eryyey + 75 gm. 
peeatiemenmoater to make’. (ici. lee eee eos By) OZ: 1000 cc. 


For use take equal parts. 
The following formula is suitable for general work, giving contrast 
more readily owing to the presence of hydrochinon: 


Finding vs ow 8445 ve cis ses es 4O er. 405 gm. 
OE EES ae a eee 120 er. 14.0 gm. 
Bete? Sialite Cdty) oes eco c see t case u ee 240 gr. 27.5 gm. 
ES a eee 20 gr. 2.4 Pin. 
ROI CIRO Se laces vv bie he cede eee oo 20 02. TO0G)4,, CC. 

PM OS HEIDE DPOUMCC. os cas Sc. c ee ee ei eee 5 er. 0.5 gm. 
emi ti eat OONGle (OLY) % aise ee ss ese seca ea 30 er. 3.5 gm. 
CPU N NER ERL oa lis oicia. 4's ors o s-+ « wlese SAAS 30 Oz. 1000 CC. 


For use take equal parts of I and IT. 
Glycin.— Para-oxy-phenyl-glycine, C,H,OHNHCH,CO,H: 


—OH 
rao 


NH.CH2,COOH 


Glycin is a grayish, crystalline powder which is practically insoluble 
in water but is readily dissolved in alkaline solutions. The solubility 
at 15° C. in pure water is only 0.23 gram per liter while the solubil- 
ity in a five per cent solution of sodium sulphite and an equal amount 


304 PHOTOGRAPHY 


of sodium carbonate is 12.8 grams per liter. As a developer it is slow 
but powerful and gives extremely fine-grained negatives of a gray- 
black color. While glycin may be used in combination with pyro or 
metol it is generally used alone. It is particularly suited to copies of 
black and white subjects where its fine-grained images, density-giving 
power and clean-working properties make it eminently suited. It is 
an excellent developer for use in tanks, owing to its good keeping 
qualities. 
For a glycin tank formula the following is reliable: 


Boiling water... 1.4 64.00. ss pee 23 eee A De 1000 Cc. 
Sodium  sulphite Gdry) 2. ./cos%% 4s es «te ae 1% oz. 315 gm. 
When dissolved add: 

Glycift 04 sake sO dada dae ca ee le ee ae 250 gm. 
And then in small quantities: ‘ 
Potassium carbonate,. +00... <1 oie fre 8 1250 gin. 


This forms a thick cream which must be well shaken before dilut- 
ing’ with water. For all normal work take 1 part of the above to 
twelve to fifteen parts of water. For very soft results or for slow 
tank development dilute with 20-30 parts of water. 

For a more energetic developer combining speed and also the good 
points of glycin, a metol glycin formula as that which follows is suit- 
able : 


T. GIvCID | ue a be apy we oe ewes ee ne 60 gr. 6.8 gm. 
Sodium. sulphite: (dry)<.0¥..055 see 1% OZ. 25 gm. 
Metol iaies ns 9s ns eee ae ee reso 40 gr. 4.5 gm. 
Citric acid 5 Se ee ee ee 20 gr. 2.3 gm. | 
Potassium bromides. 4.5 . case ss eee 13° gis 1.5 gm. 
Water to: make. csv i. vis scan 6 te eee ON 2 ge: 1000 CC. 

II. Sodium ‘carbonate (dry)... ..2% peu Lie0m 50 «gm. 
Water ‘to make.o.4 5/7004. hs ee 20 or 1000 CC. 


For use take equal parts. 
Hydrochinon.— Para-dioxybenzene, C,H (OH),: 


ne 


& 


| 
OH 


ORGANIC DEVELOPING AGENTS 305 


Hydrochinon is one of the oldest developers which is now in gen- 
eral use, its developing properties having been discovered by Abney in 
1880. Chemically it is dioxybenzene, the two hydroxyl groups being 
in the first and fourth, or para, position. The commercial product 
consists of long prisms or hexagon needles either pure white or nearly 
so. The solubility in pure water at 15° C. (59° F.) is 5.7 grams per 
liter and in a five per cent solution of sodium sulphite and an equal 
amount of sodium carbonate the solubility is 7.4 grams per liter. It 
dissolves more readily in hot water than in cold or may be dissolved 
in alcohol quite readily. Hydrochinon is very sensitive to low tem- 
perature and also to potassium bromide both of which restrain its ac- 
tion considerably. Thus at 40° F. (5° C.) hydrochinon is practically 
inert while at temperatures below 60° F. (15° C.) there is a loss in 
both density and detail. It is therefore important that hydrochinon 
developers be kept as near as possible to a normal temperature of 65° 
F. or approximately 19° C. The same caution also applies to com- 
binations of hydrochinon and other developing agents. The restrain- 
ing action of potassium bromide is considerable and it must be added 
with caution, the maximum amount being approximately five drops of 
ten per cent solution (0.5 grain) per ounce of mixed developer. The 
proportion of sodium sulphite also has a slight influence on the con- 
trast, larger amounts giving increased contrast. 

Hydrochinon is rather slow in action, unless caustic alkalis are used, 
and tends to give negatives with a good deal of contrast. The use of 
caustic alkalis in place of the usual alkaline carbonates gives a more 
energetic developer which works more softly. The chief use of hy- 
drochinon alone is in reproduction work such as copies of line draw- 
ings and similar work in which the maximum contrast is desired. For 
general use it is combined with ortol, metol, eikonogen or one of the 
rapid, soft working developers. 

According to Crabtree the following is the best formula for maxi- 
mum contrast yet devised: 


Pe ati IGMIDINITE i kk aes cad es eee ee sie S950. oF, 25 gm. 
BCE Ce REY a i, eS vcha aa ood tives ae we 375 4 2K. 25 gm. 
yO) Sig eens (ih rer 47h er. 25 gm. 
NE IRIE oy. vile ee ete scene teva ha es “Pi yewe: a IOOO Cc. 

Ned cy cics on id wey whee sk Cada es 14 oz. 45 gm. 
AO iss occu eu eva sana es new es SE's 62, 1000 CC. 


For use take equal parts. Hydrochinon combined with metol forms 
a very versatile developing solution, the metol supplying the speed and 


306 PHOTOGRAPHY 


detail while hydrochinon builds up the necessary density. This com- 
bination forms perhaps the most popular developer in existence and 
formulas are without number. For plates and films nothing is su- 
perior to the MO formula of Alfred Watkins as used in the thermo 
system. . 


Li Aydrochinon (4/36. 00... 556 va 00)? ee 2I gm. 
Metol cio. se tbececsaan eae eae rene 30.—s grr. 7 gm. 
Sulphite of soda : (dry) 13.22 Ae) eee te ROG 100 gm. 
Water to make... ¢cus. ge vae a IOs) i, O2. 1000 cc. 

Ii; Sodium’ carbonate (dry). 2225..95> en eee 14 oz. 275 gm. 
Water to make: .5:3 .7sas tus sas ee 10 Oz. 1000 Cc. 


For use take 34 dram each of I and II and make up to 1 ounce with 
water. In metric measure 94 cc. of each to 1000 of water. The Wat- 
kins factor is 15. Further dilutions are given in the following chapter. 

The action of the desensitizer phenosafranine on hydrochinon is such 
that it raises it from a slow, contrast working agent to one of high 
speed and tending to softness in much the same way as metol. Safra- 
nine has therefore been suggested by Dr. Luppo-Cramer as a cheap 
substitute for metol. He gives the following hydrochinon-safranine 
formula: 


A. Hydrochinon 2.5 cae, ies ee 12 gm. 52.5 gr. 
Sodium gulphite (dry) ..........60.-s0000 40 9 OO poem ee 
Potassium: bromide... .."...5....4s.0<etee ae I gm. 4.4 gr. 
Water to. makes.) ii Hbasl eet ee 1000 cc. IO Oz. 

B. Potassium carbotiate.+ 2.2...) «sc. oes = eee 50 gm. 219 gr. 
Phenosafranine I: 2000 sol........... Suile eae 200 CC. 2 oz. 
Water :to make.ci.dic50 ca ee iemdale 1000 cc. IO... OZ, 


For use take equal parts of 4 and B. In mixing the B solution do 
not add the dry carbonate to the phenosafranine.® 

Metol.— Mono-methyl-paramidophenol-sulphate, C,H,OHNHCH, 
+ (H,SO,) : 


OH 
H,SO, 
+ ea 
NH.—CHs3 
Metol base 


® Formulas for developers of the Rodinal type and Hubl’s glycin paste con- 
taining phenosafranine have been given by the same authority in Der. Phot., 
1921, 31, 193; S. JT. J. P., 1921, 1, 69; Phot. Ind., 1921, p. 534. 


| 
i 
: 
3 
j 
1 
; 


ORGANIC DEVELOPING AGENTS 307 


Metol was introduced in 1891 by Hauff, its develoing power having 
been observed by Bogish. It seems that metol as first issued was the 
sulphuric acid salt of di-methyl-paramido-meta-cresol. Apparently 
the cresol base was abandoned at a later date in favor of phenol, since 
both Hauff and Andresen described the same as mono-methyl-par- 
amidophenol sulphate. For a long time nothing definite was known 
regarding its structure or manufacture since the patents, which were 
at that time the only source of information, cover its use as a de- 
veloper only and tell nothing regarding its structure. Lately, how- 
ever, Mr. W. F. A. Ermen, a British chemist, has been able to prepare 
metol which is not only equal in every way to that of German origin 
but has the added advantage that the substance producing metol 
poisoning has been isolated and removed without affecting in any 
way the developing properties of the substance. For a full account of 
this very valuable paper reference should be made to the Photographic 
Journal, 1923, 63, 223. 

The commercial product is a white or grayish white powder readily 
soluble in water. The solubility at 15° C. (59° F.) is 4.8 grams per 
liter while in a five per cent solution of sodium sulphite with an equal 
amount of sodium carbonate the solubility is 4.5 grams per liter. The 
solubility increases considerably as the temperature is raised. Metol 
is one of the most conspicuous of the class of soft-working, rapid de- 
velopers, the image appearing very quickly and in full detail while 
density is added slowly so that considerably longer development is 
necessary than would be judged to be the case from the rapid appear- 
ance of the image. 

Metol is seldom used alone but generally in combination with hy- 
drochinon in order to secure greater density and contrast. Although 
metol is more energetic than hydrochinon, in practice a combination 
of metol and hydrochinon is more rapid than either alone. This is 
due to the fact that metol brings out the details of the image very 
quickly while hydrochinon adds the density and contrast required to 
secure satisfactory printing quality. 

Metol is able to develop without the addition of alkali and this fact 
is often taken advantage of to develop difficult interiors and other 
subjects where extreme contrast and halation are unavoidable. The 
following formula may be recommended : 


eels a gs sn ane Sy ae Win amen Goals acai 40 Oz. 1000 CC. 
RG vb irra vee Perea wean rays alses 50 gr. 2.5 gm. 
MRT a a na oe bs od Sa seers LO ewes 240 gr. I2 gm. 


Bora emipnite (dry)... cc. ccs voce eve cece teeacs 960 gr. 48 gm. 


308 PHOTOGRAPHY 


For use dilute one part of the above with seven parts of water. 
Development is rather slow, owing to the absence of alkali, and will 
require from 20-30 minutes at 70° F. (21° C.). Small amounts of 
a solution of carbonate of soda may be added with advantage towards 
the completion of development. Combinations of metol and . hy- 
drochinon, metol and pyro, metol and ortol will be treated under the 
heads of pyro, hydrochinon and ortol respectively. 

Metoquinone.—A mixture of metol and hydrochinon, (C,H,OH- 
NH) (CH;)2(C.H,(OH),: 


H 
(CoH + CsH.(OH)? 
NH(CH8)? 


Metoquinone is a chemical combination of metol and hydrochinon 
introduced by Lumiére in 1903 and may be used with or without an 
alkali. It is a fine white powder soluble with difficulty in cold water 
but freely soluble in hot. The following is Lumiére’s formula for 
use of the same: 


Water (hot) \cccacel)s 5 \san sich vienna eee 20 oz 1000 cc. 
Metoaqttinone ios) sss14.easleuhcoee a ntes eee 85s gr. 8.5 gm. 
sodium: sulphite Cdry)s spss 05's «nn + os eee 1% oz. 62.5 gm. 


For the development of instantaneous exposures add: 
Acetone 3. 83.5 ee 1% oz. 62.5 Cc. 


Monomet.—Para-amido-o r t h o-c res 0 l-hydrochloride, HOC,H,- 
(CH,)NH,HCl. Monomet is a British product introduced by the 
White Band Company in 1916 to replace German metol then no longer 
obtainable on account of the war. In appearance it is a grayish white 
powder which keeps well in solution as well as in the dry state. Its 
solubility is less than that of metol but is sufficient to allow fairly 
concentrated solutions to be prepared. Like metol it is free from 
stain and rapid in action but unlike metol it gives a pure black rather 
than a blue-black deposit. It is non-poisonous and has no effect on 
the skin. It may be substituted in any formula calling for metol by 
substituting for metol weight for weight, but the amount of monomet 
should not be greater than 1% grains per ounce or it will be impos- 
sible to keep the same in solution. | 

Neol.—The latest of the organic developing agents, Neol, is stated 
to be para-aminosalicylic acid and was introduced by Hauff in 1918.1 


10 British Patent No. 154,198 of 1918. 


| 
| 


- ORGANIC DEVELOPING AGENTS 309 


Neol requires to be used with a caustic alkali and it is claimed that it 
is without. fogging or staining action and that it allows of correction 
for over exposure up to 100 times. The correction is attributed to a 
pronounced tanning of the film by the substance formed as a result 
of the reaction between the developer and the latent image. Dr. 
Luppo-Cramer expresses doubts as to the theory and also the exist- 
ence of any specific corrective action and comes to the conclusion that 
Neol offers no advantages over other developing agents.’ 
It is prepared in the following stock solution: 


A ES oce ie aha < 4.4 sidildis to hss cca did alae ss 100 gm. 90 gr. 
Prodtiim stipe (dry) in. ccc ce ces ecw 500 gm. 454.5 er. 

(A) Caustic lye (200 gm. pure caustic soda per 
TE ad oy See 210 cc. 110.3 min. 
NI ES Gh ocois ii ps 51s Wy ae s oes oe ee es 4790 CC. TO ue OZ. 
TT VOTOXIGE «onc ccs cn cwlew uc ewavase 200 gm. 202. 
By TIENT AON 9 yh s)sis cee oie die via b. © Laie vlore <a 1000 cc. 1 = Oe 


The developing solution is prepared according to requirements as 
follows: 


1. Over exposure or slow action: 

A 50 parts, water 50 parts, normal caustic soda 3 parts. 
2. For normal exposure and rapid development: 

A 50 parts, water 50 parts, normal caustic. soda 7 parts. 


Ortol.—Sulphate of mono-methyl-ortho-amidophenol with hydro- 
chinon, C,H,(OH), + C,H,OHNHCH, + (H,SO,/2) : 


OH OH 
—NH 
+4. | + Pe (Andresen) 
—CH; 4 
OH 


Ortol is a combination of hydrochinon and methyl-ortho ami- 
dophenol and was introduced by Hauff in 1896. Commercially it is 
supplied as a rather coarse yellowish powder which is easily oxidized, 
so that it should be kept in tightly corked bottles and in a dry place. 
The mixed solution however has good keeping qualities if potassium 
metabisulphite is used as a preservative. Ortol forms an excellent 
developer either alone or in combination. Used alone it is slower than 


11 Phot. Korr., 1920, p. 270. 


310 PHOTOGRAPHY 


metol and paramidophenol, but not so slow as glycin or hydrochinon 
and its general properties fall between these two classes of developer. 
It is used with alkaline carbonates and is rather sensitive to potassium 
bromide which must be used with caution. The color of the image is 
brown-black, very similar to that of a pyro developed negative al- 
though not so yellow and totally different to the blue-black of most 
other developing agents. It is non-staining and has practically no 
tendency to cause fog. The following formula is advised: 


Dv Ortot 54s ca Nicene eee eS heLSe CeeeD 140 gr. 15 gm. 
Potassium metabisulphite................... 70 gt. 8 gm. 
Water. to. maketic.s deen eee 20 OZ. 1000 cc. 

If. Sodium. carbonate (dry) ye ee ee 1% oz. 65 gm. 
Sodium sulphite. (dry) s\.c4..de2s ae ae 134 Oz. 85 gm. 
Water to make... iii ci lsavas eee eee 20:.° Os: 1000 cc. 


Under certain conditions it may be advisable to add from 10 to 20 
grains (1.I-2.3 grams) of potassium bromide to solution No. II. 
For use take equal parts. For a slower developer add one part of 
water. 

The following formula is very suitable for tank use: 


Potassium metabisulphite........ 4.4. +1458 cee 5 gr. 5 gm. 
Ortol 5 o:i.0:ia wa vivis eid aaa stele a a eee 10 gr. 1.0 gm. 
Sodium sulphite (dry) .<. us. ss aun ee 30 gr. 5 om 
Sodium “carbonate | (dry). 023). ..cineentn sae 30 gr. 3 gm. 
Water oc cia ssc waclan o's wy eiuc one mines see 20 02. 1000 cc. 


With plates listed as M by Watkins this will develop in 20 minutes at 
65 E. 7 
Paramidophenol.—F ree base—C,H,OHNH,: 


OH 
i 


\ 


NH, 


Paramidophenol differs from hydrochinon in that an amido group 
is substituted in the fourth position in place of the hydroxyl group. 


eS ee ee OE ee ere oe eee 


50 FF aus 


ye ee. ee eS ee 


——— Oo ee 


(ee eS eee eee ee ee ae Ye ee ae ee 


ORGANIC DEVELOPING AGENTS 311 


a6 


Hydrochinon Paramidophenol 


The free base is so sparingly soluble in water that it is of little prac- 
tical value and accordingly the hydrochloride or sulphate is generally 
used. ‘The first takes the form of fine prism-like crystals, white or 
nearly white and readily soluble in water. In water at 15° C. (59° 
F.) the solubility of paramidophenol hydrochloride is 33 grams per 
liter, while in a 5 per cent solution of sodium sulphite plus an equal 
amount of sodium carbonate the solubility is only 3.2 grams. 

When made up with sodium sulphite and an alkaline carbonate the 
free base is liberated and there is a gradual loss in developing energy. 
The addition of very small quantities of sodium hydrate (caustic 
soda) will redissolve the precipitated paramidophenol base and keep 
the energy of the developer up to normal. Paramidophenol hydro- 
chloride is very sensitive to the action of potassium bromide which 
should exceed one part to ten of paramidophenol only in exceptional 
circumstances. 

The sulphate salt is in general very similar to the hydrochloride, 
being perhaps less energetic, but only to a very small degree. As with 
the hydrochloride salt there is a precipitation of the free base in the 
developing solution and the addition of small amounts of caustic soda 
from time to time is required to keep the developing power up. 

Either the hydrochloride or sulphate salts of paramidophenol form 
clean, rapid-working developers which give soft images with plenty of 
detail but produce density only when development is prolonged. In | 
its various forms it is one of the most popular developers in use. 

A typical formula follows: 


Pent eRIOONETION oY sad ee ea oe ds 200 ‘gr. 23.0 gm. 
Potassium metabisulphite.................. 100 gr. 11.5 gm. 
ERTIES BPEL oes an eee wea tcdctan teense 20.- 02. 1000 CC. 

Pimocuium sulpnite (dry)... 2.01.05. ce ee eee 34 Oz. 30 gm. 
Porasetumucarponate (dry) i....<s..-0.+.- 14 oz. 60 gm. 
RE eS SIE oy ota de Waid as ppe wine Wis «s,s 20 Oz. 1000 CC. 


312 PHOTOGRAPHY 


For use take one part of No. I to two of No. II.” 

Paramidophenol; Phenolate Compounds.—Towards strong alkalis 
paramidophenol acts as an acid, therefore if the hydrogen atom of 
the hydroxyl group is replaced by an alkali metal such as sodium, a 
phenolate compound results having in this case the formula C,H,- 
ONaNH, or sodium paramidophenolate. This is actually the sub- 
stance formed when a solution of caustic soda is added to a par- 
amidophenol developer in order to redissolve the precipitated free 
base. Advantage is taken of this fact to prepare highly concentrated 
solutions of paramidophenol which simply require dilution with water 
in order to be ready for immediate use. It is in such form that par- 
amidophenol has achieved its greatest popularity under the names of 
Rodinal, Citol, Azol, Activol, Certinal, Paranol and Kalogen. They 
are all patented substances but a developer of similar compos can 
be prepared by the formula to follow: 

Bring to a boil 250 cc. of pure water and, when just atte the 
boiling point, add a few crystals of potassium metabisulphite and when 
these are dissolved 20 grams of paramidophenol hydrochloride and fin- 
ally 60 grams of potassium metabisulphite. The mixture is stirred 
until all of the metabisulphite has dissolved, and there is then added, 
with constant stirring, liquid commercial caustic soda sufficient to re- 
dissolve the amidophenol base. The mixture becomes thick at first 
owing to the precipitation of the amidophenol base and as the caustic 
soda is added gradually clears up owing to the precipated base going 
into solution. The addition of caustic soda should be stopped just 
before all of the paramidophenol base is dissolved and the solution 
made up to 400 cc., placed in a rubber-stoppered bottle and allowed 
to cool. It is very important that a small quantity of paramidophenol 
base be left undissolved as the least excess of caustic soda causes the 
solution to be unstable, rapidly turning brown and losing developing 
power. ‘This is the only trick in the operation and must be carefully 
observed. A solution properly prepared will keep almost indefinitely. 
Should an excess of caustic soda be added by mistake, the matter may 
be remedied by the addition of a solution of sodium bisulphite until 
a slight precipitate of paramidophenol is formed. 

12 No addition of caustic soda is required with methylated paramidophenol, 
the use of alkaline carbonates alone producing an energetic developer very similar 


to metol. 
13 Metol and Monomet may also be used to prepare developers of this char- 


acter. For full directions see Ermen, Brit. J. Phot., 1920, 67, 611; see also 


pp. 610-611. 


a 
4 

5 
4 
: 
- 
: 
a 


ORGANIC DEVELOPING AGENTS 313 


Pyrocatechin.—Ortho-dihydroxy-benzene, C,H,(OH),: 


OH 
—OH 
—OH 
| OH 
Pyrocatechin Hydrochinon 


Pyrocatechin is very closely associated chemically to hydrochinon 
which is also a dihydroxyl. Hydrochinon, however, is a para de- 
rivative while pyrocatechin is an ortho derivative. That is to say, 
in the former case the hydroxyl groups occupy the first and fourth 
points of substitution and in the latter the first and second positions. 

Pyrocatechin is a prismatic, colorless, crystalline substance freely 
soluble in water. The solubility in water is 33.3 grams per liter at 
ordinary temperature and in a five per cent solution of sodium sul- 
phite with an equal amount of carbonate the solubility is 35.7 grams 
per liter. 

Alkaline solutions of pyrocatechin were advised as a developer by 
Eder and Toch in 1880, the same year that Abney discovered the de- 
veloping power of hydrochinon. As a developer pyrocatechin is 
more energetic than hydrochinon and less sensitive to temperature. 
It differs from most organic developers in that it retains its normal 
developing power in the presence of “hypo” and this property has 
been utilized in processes of simultaneous developing and fixing. 

Pyrocatechin was formerly sold under a variety of trade names 
such as Kachin, Elconal, etc. These have now disappeared but pyro- 
catechin is still obtainable. 

A typical formula follows: 


OME POACHING ho bee cs ole Use ndaae ue ee 175°) gr. 20 gm. 
Bocmsurescinnite Cdry) . 2 sciee evs cs cae ress 4, OZ. 37.5 gm. 
PBC TAO 5 ise sieves «ae gig base evens 20 Oz. 1000 cc. 

eee ARS TATOONATC . 5s a cs vis'sae eae ne cas 24 oz. 125. gm. 


URE ERP MEINORY GS tak scx wig bok OE fied eek ore 20 OZ. 1000 cc. 


For use take equal parts. 
Pyrogallol; Pyro; Pyrogallic Acid.—Tri-oxy-benzene, C,H,- 
(OH),: 


314 PHOTOGRAPHY 


OH 


te 
Jon 


Pyro is the oldest of the organic developers and still one of the most 
popular. The use of pyro dates from the publication of the wet col- 
lodion process by Scott Archer in 1851 but it was not until 1862 that 
Major Russel advocated its use in an alkaline solution. When the 
gelatine dry plate was introduced in 1874-5, alkaline pyro was found 
to be an ideal developer for the same and from that day to this pyro- 
gallol has held its place in popular esteem regardless of the numerous 
agents which have since been discovered and placed upon the market. 
Even to-day, despite the number of other developing agents, pyro is 
perhaps in more general use than any other developing agent. 

As a developer pyro will do practically anything that any other de- 
veloping agent will do. Used in weak solutions it is a soft-working 
developer, while if used in a concentrated form it is capable of pro- 
ducing a high degree of contrast. By the proper modification of the 
strength of the developing solution it is therefore possible to secure a 
developing solution adapted to the requirements of the particular sub- 
ject in hand. Pyrogallol possesses the disadvantage of having poor 
keeping qualities and of staining both the negatives and the figures. 
The amount of the stain is directly under the control of the worker 
who by proper adjustment of the proportion of sodium sulphite may 
secure just the degree of stain which he desires. Owing to the rapid 
oxidation of pyro in solution it is well to take fresh solution for each 
plate but even so pyro is the most inexpensive developer in use. Stock 
solutions keep well when a small amount of an acid or sodium bisul- 
phite or potassium metabisulphite is added as a preservative. 

Pyrogallol is readily soluble in water, its solubility surpassing that of 
any other organic developing agent, being 52.4 grams per liter in water 
at ordinary temperature. Ina 5 per cent solution of sodium sulphite 
with an equal amount of carbonate the solubility is 41.8 grams per 
liter. It is very sensitive to the action of potassium bromide which 
should rarely exceed 0.5 grain per ounce. 

Formulas for pyro are without number and may be found in the 
instruction sheets of every plate or film manufacturer and, while it is 
advisable to use the formula recommended by the manufacturer, the 


ORGANIC DEVELOPING AGENTS 315 


following formula is well adapted to practically every brand of plate 
or film on the market. It is the Watkins Thermo system formula: 


I eee aes hare kes tay ees os 160 gr. 36.5 gm. 
Metapisuiphite of potash... ........6.0. 00008. 80 gr. 18 gm. 
eMC RTENOOA Se Pcs OES L A Teles a cees 2 Oz. 200 gm. 
ARE ORES ANIA soho. alc tis A) sled Meain se ven eos IO Oz. 1000 cc. 

emsarnonate of soda (dry)... ....00....2.../... 2 Oz. 200 gm. 
eet TAT DERITIIG ss oinceaiciale salece dle we cwie'e es 40 gr. 10 gm. 
a SU RR Vos eis ea vip adie ciajdin eo ass 10 Oz. 1000 cc. 


For use take one dram of each and make up to 1 ounce of solution. 
In metric measure 125 cc. of each to 1000 cc. of water. The Watkins 
factor for the above with bromide is 5: without bromide 12. 

Minor Organic Developing Agents.— Under this heading are classi- 
fied several compounds used for developing which are not generally 
used or are no longer obtainable on the open market in this country. 

Diogen and Imogen were two introductions of the Agfa Company 
which are no longer obtainable in this country. The chemical formula 
of the former was stated to be sodium-alpha-amido-naphthol-disul- 
phonic acid, while the composition of the latter was not made public. 

Duratol was said to be benzyl-paramidophenol. It was introduced 
by Schering about 1910 and was for a time quite popular in this coun- 
try. 


OH 
a 


NH.C.H; 
Duratol 


As a developer Duratol is very similar to metol. It is comparatively 
rapid in action, non-staining, non-fogging, very stable in solution, and 
not so easily exhausted. In combination with hydrochinon it forms an 
ideal developer for either plates and film or for bromide or gaslight 
papers. 

Chloranol is a combination of hydrochinon and methyl-paramido- 
phenol introduced by Lumiére. The formula is 


OH (1) 2 OH (1) 
jew | i CoH 
_ \NH(CH#) (4) OH (4) 


Hydramine, a chemical combination of hydrochinon and_para- 


316 PHOTOGRAPHY 


phenylene-diamine, was introduced by Lumiére. ‘The developing 
properties of para-phenylene-diamine were observed by Andresen in 
1888 and it is stated that Hauff made, but never placed upon the market 
commercially, the combination of hydrochinon and para-phenylene- 
diamine which Lumiere, at a later date, introduced as Hydramine. It 
is supplied as a white powder, readily soluble in water and in conjunc- 
tion with caustic alkali and sodium sulphite produces a slow-working 
and non-staining developer which keeps well and has no injurious 
effect on the skin. ; 

Pyraminol, the condensation product of hydrochinon and paramido- 
phenol, was introduced by Hauberrisser a few years ago but is now no 
longer on the market. It was very similar as a developer to paramido- 
phenol. 

GENERAL REFERENCE Works 
Eper—Ausfihrliches Handbuch der Photographie. 


SEYEWETZ—Le Negatif en Photographie. 
VALENTA—Photographische Chemie und Chemikalienkunde. 


ae 


ea Cw ee A? ep ele ew fee ey Pie Sm 


CHAPTER XIT 


THE TECHNIQUE OF DEVELOPMENT 


Introduction.—The chemical and physico-chemical basis of the 
process of development and the chemical and photographic properties 
of the organic compounds used for development have formed the sub- 
ject of the two preceding chapters. In this, the last chapter on the 
subject, we will be concerned for the most part with the more prac- 
tical aspects of the matter—the technique of development. 

The developing solution as applied to the plate consists of four in- 
gredients: the developing agent itself; the preservative, generally 
sodium sulphite ; the alkali, one of the alkaline carbonates usually ; and 
the restrainer, potassium bromide. Amidol is an apparent exception 
to this as no alkali is added, but it is probable that the hydrolysis of 
the sulphite which is always used furnishes the sodium to form a 
phenolate which is the actual developing agent. ‘Then again the re- 
strainer is quite often omitted, particularly in negative developing 
solutions, but generally we may say that the developing solution con- 
sists of the developing agent, the preservative, the alkali, and the 
restrainer. The part played by each of these in the process of de- 
velopment has already been considered ; there remains, however, a few 
matters of practical importance regarding the use of the sulphites and 
alkalis in development. These matters it is proposed to take up in the 
present chapter, together with desensitizing as applied to photographic 
development, and finally the three principal methods of poco: 
inspection, factorial and thermo. 

The Sulphites in Development.—The use of sodium sulphite as a 
preservative in solutions of the organic developing agents appears 
to have been first suggested by Berkeley in 1882.1 The theory of its 
action has already been discussed (page 270). 

There are two forms of sodium sulphite in general use, the an- 
hydrous and the crystalline, the latter containing seven molecules of 


1 Phot, J., 1882, 22; Brit. J. Phot., 1882, 29, 47; Phot. News, 1882, 26, 41. 
22 317 


318 PHOTOGRAPHY 


water of crystallization having the formula Na,SO,:7H,O. Calcu- 
lating the molecular weights of the two forms we have 


Na, +CO, ==126 
Na.CO,-7H,U ==50 


Thus 126 parts of the anhydrous are equivalent to 252 of the crystal- 
line salt, or, in other words, the anhydrous form is just exactly twice 
as strong as the crystal. Hence when using anhydrous sulphite in a 
solution calling for crystals only one half of the amount called for by 
the formula should be used. As the crystal form is in almost uni- 
versal use in England whereas the anyhydrous is in general use in 
this country, this fact should be borne in mind when cee use of 
formulas from an English source. 

No matter which salt is used it is rare to find it pure, and com- 
mercial varieties are likely to contain from 2 to as much as I0 per 
cent of impurities principally as sulphates or as carbonates. While 
theoretically one ought to test each batch, this is unnecessary since 
an excess is always used. Of the two forms the anhydrous keeps 
better in the dry state and for this reason is to be preferred over the 
crystalline form. 

Stock solutions of sulphite do not keep well and it is advisable to 
prepare at one time no more than it is expected to use within a week 
to ten days. Specially prepared solutions of sulphite containing al- 
cohol or potassium metabisulphite, however, may be kept much longer. 
The addition of 10 per cent of alcohol has a beneficial effect on the 
keeping properties while the so-called “neutral sulphite” is even 
more effective. Most commercial sulphite is slightly alkaline and 
this affects the keeping properties, hence the addition of just sufficient 
acid to neutralize the sulphite is often advised. Sulphuric is the best 
of the strong acids for this purpose although oxalic and citric acid are 
often used. Perhaps the most effective means of preserving sodium 
sulphite solutions is by the use of potassium metabisulphite as follows: 


Sodium sulphite dry eis ois ccseu ovens ee eee ee 2. em 100 gm. 
Potassium metabisulphite.............-.+.0ceese IY oz. 25 gm. 
Water to make... .... Ps cb ta ee oo eee 20 «Oz. 1000 cc. 


_ Dissolve at ordinary temperature, then raise to the boiling point and 
finally allow to cool.? 
2 The addition of small quantities of hydrochinon as a preservative of stock 


solutions of sulphite has recently been suggested. See Journ. Camera Club of 
London, 1923, I, 3. 


Re ee ae a ee eee 


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——- be 


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. 
. 
; 


THE TECHNIQUE OF DEVELOPMENT 319 


In place of sodium sulphite the corresponding potassium and am- 
monium sulphites have been recommended, as have also potassium 
metabisulphite and sodium bisulphite lye, but owing to the cheapness 
and efficiency of sodium sulphite none of these compounds have ever 
been widely adopted.’ 

In the case of a developing agent producing a stain image, the pro- 
portion of sulphite controls the intensity of the stain. Thus in the 
case of pyro, increasing the amount of sulphite will decrease the 
amount of stain to a point where the image is almost pure black, while 
if the sulphite is decreased the intensity of the stain will gradually 
increase, passing from black to warm-black, and finally to yellowish- 
brown. With non-staining developers the proportion of sulphite does 
not have any decided influence on the color of the deposit, acting prin- 
cipally as a preservative of the developing solution. 

The Alkalis in Development.—When alkaline development was first 
introduced by Russel in 1862 the alkali in common use was ammonia 
and pyro-ammonia continued to be the favorite developer for many 
years. Even as late as 1900 pyro-ammonia was still considerably 
used for negative work. Owing, however, to its volatile nature its 
action is erratic and uncertain and at present ammonia has been com- - 
pletely replaced in all but a few instances by the fixed alkalis such as 
the alkaline carbonates and hydroxides. 

Of these the carbonates are the most widely used, particularly 
sodium carbonate. Potassium carbonate is used in some special cases, 


while the hydroxides are occasionally used with hydrochinon and 


elycin, or for the production of phenolate compounds from par- 


~ amidophenol. 7 


There are three carbonates of sodium, the bicarbonate or acid car- 
bonate, the sesqui-carbonate and the normal carbonate. Only the lat- 
ter is suitable for development. The normal carbonate exists in three 
forms, the anhydrous NaCO,, the monhydrate containing one mole- 
cule of water and having the formula NaCO,; -H.O and the crystalline 
having ten molecules of water of crystallization and consequently be- 
ing NaCO,:10H,O. Calculating the molecular weights of the three 
forms: 

3 Ammonium sulphite, Eder, Phot. Korr., 1885, 22, 111. Potassium metabi- 


sulphite, Mawson and Swan, Brit. J. Almanac, 1887, p. 139; 1888, pp. 316 and 346. 
Sodium bisulphite lye, Gilder, B, J. Almanac, 1891, p. 718. 


320 PHOTOGRAPHY 


Crystalline NaCO, : 10H,O 
46 + 60 + 180 = 286, 
Monohydrate NaCO,-H,O | 


46-+ 60+ 18 =124, : 
Anhydrous NaCO, 
46 + 60 == 106. 


Consequently we see that 106 parts of anhydrous sodium carbonate 
are equal to 124 of the monohydrate and 286 of the crystalline form. 
The anhydrous carbonate is therefore approximately 2.8 times as ef- 
ficient as the crystalline. As a matter of fact, however, the pure an- 
hydrous carbonate is not often met with, the usual dry or so-called 
‘anhydrous carbonate” being in reality the monohydrate, NaCO,- 
-H,O. This is slightly stronger by twice than the crystalline salt as 
286 : 124 :: 100 : 43.4. It is quite accurate enough, however, for all 
practical purposes to take the efficiency of the ordinary dry sodium 
carbonate as being twice that of the crystalline form. The monohy- 
drate is generally used in this country, the crystalline form, however, 
is still used in England—a fact which must be borne in mind when 
using formulas from that country, as, unless specified as dry, the 
quantities always represent crystal carbonate. 

The monohydrate is to be preferred to the crystalline salt owing 
to the fact that it is more stable, as it does not effloresce or lose its 
water of crystallization. The crystal carbonate owing to its larger 
water content is also more apt to absorb carbon dioxide from the air. 
This unites with the carbonate to form bicarbonate, a substance of no 
practical value in development. 

There is no settled proportion of alkali which should be used with 
any given plate or developing agent. Variations within reason have 
no other practical effect than increasing or decreasing the velocity of 
development. The existence of an optimum concentration of alkali 
beyond which no further increase in velocity occurs when the alkali 
is increased has been shown by Ermen.* Except in the case of hy- 
drochinon, this optimum concentration equals about 1 per cent of the 
anhydrous salt. Increasing the carbonate above this point does not 
increase either the velocity of the developer, nor the density, but 
probably does increase the amount of fog and for this reason should 
be avoided. 

Not much use is made of the caustic aici such as the sodium and 


4 Phot. J., 1922, 62, 123. 


THE TECHNIQUE OF DEVELOPMENT 321 


potassium hydroxides, except for the preparation of paramidophenol 
developers and with such slow acting agents as hydrochinon and glycin. 
When used with hydrochinon, the caustic alkalis form a more active 
developer, which works more rapidly and softer than that produced by 
the use of the alkaline carbonates and one which is less affected by 
temperature. Owing to their supposed tendency to fog and to their 
action on gelatine there exists a certain prejudice in the minds of 
many photographers against the use of the caustic alkalis. While with 
the more energetic developing agents no real advantage attends the 
use of the same over the alkaline carbonates; the caustic alkalis may 
be used with complete success if care be taken to use the proper 
amount The following table shows the developing equivalence of the 
alkalis and in what proportion one should be substituted for another. 


: : Sodium Sodium Sodium Potassium 
ee an carbonate carbonate carbonate carbonate 
de y NaCOs NaCOsH:0 | NaCO:10H2O K2COs 
80 112 106 124 286 138 
I 1.100 1.325 1.550 3.575 1.725 
0.714 I 0.946 1.110 2.553 1232 
0.755 1.033 1 1.170 2.698 1.302 
0.323 0.452 0.858 Hf 2.307 E313 
0.280 0.392 0.371 0.433 I 0.483 

0.580 0.812 0.768 0.899 2.072 I 


A number of other substances have been suggested for use in place 
of the alkaline carbonates and hydroxides but few have come to be 
used extensively. Sodium tribasic phosphate was suggested by 
Lumiére in 1906 but has never been widely used. Acetone CH,- 
CO-CH, was also recommended by Lumiére and has found some 
favor, particularly in conjunction with pyro. Its action is to combine 
with the sulphite to form acetone sulphite, the sodium set free com- 
bining with the developing agent to produce the phenolate which is the 
actual reducing agent. Its principal advantages are its freedom from 
fog, somewhat less stain than when the alkaline carbonates are used 
and no softening action on the gelatine film. Pyroacetone for the 
latter reason makes a very efficient hot-weather developer. Formalde- 
hyde was introduced by Lumiere in 1898 but has not proved to be very 
efficient with any agent except hydrochinon in conjunction with which 
it produces a contrast working developer especially suitable for line 
copies and similar work in black and white.® 

5 Sodium tribasic phosphate, Eder’s Jahrb., 1896, 10, 190. Acetone, Bull. Soc. 
franc. Phot., 1896, p. 558; 1897, p. 550. Formaldehyde, Eder’s Jahrb., 1808, 12, 
419. 


De RCS” a 
ue ec 

ot ae 

Bey 

a 


322 PHOTOGRAPHY 


The Value of Desensitizers—One of the most noteworthy addi- 
tions to photographic technique during the last few years has been 
the introduction of efficient desensitizing agents which by reducing the 
sensitiveness of the plate enable development to be conducted in a 
much brighter light than that which may be used otherwise. The 
great sensitiveness of the modern dry plate has made their develop- 
ment a matter of real difficulty for no light which is sufficiently bright 
to be of much value in practice may be used without danger of fog. 
Particularly is this true for plates which are extremely sensitive to 
color, such as the modern panchromatic for which no light is really 
safe and time and temperature methods are the only satisfactory 
manner of development. The use of a desensitizing agent, however, 
enables plates of the very highest sensitiveness to be developed with 
comparative safety in a bright yellow or orange light, thus considerably 
facilitating development by either the inspection or factorial methods. 

Desensitizing Agents.—Although there are some scattered refer- 
ences in photographic literature prior to 1920 on the subject of de- 
sensitizing agents (see Wall, Amer. Phot., 1921, 15, 651, Dec.) pheno- 
safranine, introduced as a result of the investigations of Dr. Luppo- 
Cramer, was the first really practical desensitizing agent. Phenosafra- 
nine used at a concentration of I : 2000 reduced the sensitiveness of an 
extra rapid plate to 1/750 of its original sensitiveness without any 
effect whatever on the latent image so that no increase in exposure is 
required. A. and L. Lumiére and A. Seyewetz found that toluylene 
red I: 1000, aurantia I: 1000, picric acid 1: 100 are all desensitizers 
but not so efficient as phenosafranine.® 

Pinakryptol, pinakryptol green and pinakryptol yellow. are three de- 
sensitizers of unknown constitution introduced by the Farbewerke 
vorm Meister Lucius and Bruning as the result of the investigations 
of Dr. E. Konig, B. Homolka and Robert Schuloff. Both pinakryptol 
and pinakryptol green are colored substances and form highly colored 
solutions but neither has any staining action on gelatine and are conse- 
quently superior in this respect to phenosafranine which stains strongly. 
The desensitizing effect of both is as high, if not higher, than that of 
phenosafranine. Pinakryptol slows development, but with pinakryptol 
green the rate of development is unaffected. 

With both the reduction in sensitiveness is much greater for the 
blue rays than for the red so that with red-sensitive, panchromatic 
plates there is danger of fog. This difficulty has been overcome by 


6 Brit. J. Phot., 1921, 68, 351, 370. 


THE TECHNIQUE OF DEVELOPMENT 323 


the introduction of pinakryptol yellow which is a more effective de- 
sensitizer for the rays of longer wave-length. Its desensitizing power, 
however, is destroyed by sodium sulphite so that it cannot be added to 
the developing solution as can pinakryptol and pinakryptol green but 
must be used as a separate bath and followed either by a bath of 
pinakryptol green or a developing solution containing .005 per cent 
pinakryptol green. 

The investigations of the research laboratory of Pathe Cinema have 
brought out several desensitizing agents, one of which, Basic Scarlet 
N, is especially worthy of mention not only for its excellent desensitiz- 
ing properties but for its action in preventing fog... Von Hubl finds 
that Basic Scarlet N is inferior to either phenosafranine or pinakryptol 
when added to the developing solution; used as a preliminary bath, 
however, it is superior.® 

Desensitizing in Practice.—Either phenosafranine, pinakryptol, 
pinakryptol green or basic scarlet N may be used as a preliminary 
bath, or as an addition to the developing solution. When used as a 
preliminary bath, phenosafranine is used at a dilution of 1: 2000, 
pinakryptol, pinakryptol green and Basic Scarlet N at 1: 5000. An 
immersion of two minutes is sufficient, although longer immersion is 
not objectionable in the least. After this the plate may be removed 
and development conducted in a bright yellow or orange light such 
as the Wratten Series O safelight, or a suitable screen may be made 
by dyeing two fixed out plates in tartrazine (2 per cent solution) for 
thirty minutes, drying and binding up with a sheet of tissue paper 
between. 

If the desensitizer is added to the developing solution, from two to 
three minutes should be allowed to elapse before turning on the brighter 
light. According to Dr. Luppo-Cramer, the action of phenosafranine 
is complete within one minute and pinakryptol green within practically 
the same time. Pinakryptol, however, is slower in action and requires 
about two minutes and in practice it may be well to increase these 
times slightly in order to be sure that the action is completed. 

The presence of phenosafranine in the developer alters the normal 
course of development in no way except in the case of hydrochinon, 
which it raises from a slow, contrast-working agent to a rapid agent 
similar in general properties to metol. Safranine has, in fact, been 
indicated as a cheap substitute for metol in hydrochinon developers by 


7 Rev. franc. Phot., 1924, 5, 286. 
8 Phot. Ind., No. 16, 1925, p. 432. 


324 PHOTOGRAPHY 


Luppo-Cramer (see page 306 of Chapter 12). With pinakryptol 
green the rate of development is unaffected with all developing agents 
except hydrochinon, which is accelerated in much the same way as with 
phenosafranine. With pinakryptol, however, there is a diminution in 
the rate of development with all agents excepting hydrochinon with 
which there is a very slight acceleration. 

The principal objection to phenosafranine is its strong staining 
tendency. The stain produced holds tenaciously to the gelatine film 
and is extremely difficult to remove. Although it may be occasionally 
removed by prolonged washing in water it is generally necessary to 
use the following clearing bath in order to effect complete removal of 
the stain: ag 


Sodium nitrite (not nitrate)............... I gm. 4.4 gr. 
Hydrochloric -acid .(con¢,) <4..5.490 see IO cc. 0.1 oz. (fl.) 
Water tocmake, «(i canleiive vss eee ee 1000 cc. 10. Oz. 


This should not be used until the plate has received a thorough wash- 
ing. as the presence of “hypo” will lead to undesirable reactions. 
After clearing, again wash for ten to fifteen minutes before placing the 
negative out to dry. | 

Development by Inspection.—The first and perhaps the still most 
widely used method of development, certainly by the older workers, 
consists in visual examination of the plate from time to time. Deter- 
mination of the time of development by inspection is largely a matter 
of experience. It is not a method which is founded upon any definite 
scientific basis, nor one which can be expressed in terms which convey 
any exact information to another worker. Continuous experience 
under standardized conditions, and this only, will enable one to achieve 
success in developing by inspection. It is true that there are a large 
number of tips or dodges which are used by the experienced worker 
as a guide, and which are often recommended to the beginner when in 
search of assistance, but these in the absence of similar experience 
convey no exact meaning and are applicable only to those conditions 
under which they originated. Under other circumstances and in other 
hands such indications may be wide of the mark and positively mis- 
leading. | 

Actual photometric measurement of several sensitometric strips is 
sufficient to show how unreliable any attempt to estimate contrast by 
the eye is likely to be and when it is remembered that in development 
such estimation is made still more difficult by the presence of an 
opalescent film of silver halide which increases the apparent opacities, 


a © 
pe a. ae Pye a ee ee ee ee 


THE TECHNIQUE OF DEVELOPMENT 325 


but decreases the ratio of the opacities, or the contrast, it is at once 
evident that development by inspection is subject to exceedingly large 
errors and that it is a method which can be practiced successfully only 
after considerable experience with a given plate and other standardized 
conditions. 

With incorrect exposure the matter becomes still more complicated. 
The delayed appearance of the image and the slow increase in density 
and shadow detail in the case of under exposure leads to over develop- 
ment in the hope of securing greater density and more shadow detail. 
As a result the contrasts, which are already too great, are increased 
and matters are made worse instead of better. Likewise in dealing 
with over exposure the rapid appearance of the image and the quick 
growth of density lead one to remove the plate before the proper stage 
of contrast has been reached. ‘This again is just the reverse of what 
should be done as it lessens the contrasts, which owing to over ex- 
posure are already insufficient. 

While a large number of experienced photographers develop by in- 
spection, with results in every way comparable to those obtainable by 
any other method, nevertheless, it must be said that it is a more or less 
haphazard, unsystematic method of working which lacks the precision 
which is required of a process as important as that of development. 
While it is true that none of the commonly used methods of develop- 
ment are theoretically more than approximations to the required con- 
dition, nevertheless we think that it may be said that either the fac- 
torial or thermo methods are more certain and less subject to error 
than is development by inspection. It is at any rate not a method for 
the beginner, nor even for the advanced worker who works only at 
intervals and under varying conditions. For these, either the fac- 
torial or thermo methods are to be preferred. 

The Watkins System of Factorial Development.—Considerable in- 
formation concerning the velocity of development is supplied by the 
time of appearance of the image. In 1893 Mr. Alfred Watkins, the 
well-known authority on exposure and development, found that the 
time of appearance is an indication of the speed of development and 
that any variation in the dilution of the developing solution, the tem- 
perature, or the alkali, affects the time required to reach a given 
density, or value of gamma, in the same way that it affects the ap- 
pearance of the image. In other words 


Tp — WT a, 


326 PHOTOGRAPHY 


where Tp is the time for density, D, T4 the time of appearance and 
W is aconstant. It is this constant, W, which is termed the Watkins 
factor. 

With any given developing agent the factor depends upon the degree 
of contrast or gamma which it is desired to reach. Once the factor is 
found for any given set of conditions the time of development can 
always be readily determined by multiplying the time of appearance 
by the proper factor. 

What Determines the Factor.—The factor is dependent principally 
upon the developing agent. The presence of a soluble bromide, how- 
ever, has a decided effect. The factor does not change for different 
plates, except in the case of a very few plates containing a much 
larger amount of iodide than usual. Neither is it altered by varia- 
tions in the amount of the alkali nor by the dilution of the solution, 
except in the case of pyro and amidol. With these the factor varies 
with the strength of the solution. Within the range of temperatures 
generally utilized for development, there is comparatively little altera- 
tion in the Watkins factor with most developing agents. There is a 
slight variation, however, with some agents at extreme temperatures. 

The following factors are suggested for a start, but the experience 
of the photographer and the requirements of his particular printing 
medium may lead him to think an alteration of the factor desirable. 
If after the first few trials, using the factors as given, the negatives 
indicate that greater contrast would be desirable, the factor should be 
increased, while if the negatives show too much contrast a lower fac- 
tor should be used in the future. Hence the following factors should 


not be considered as final but on the contrary as suggestive only; to be ~ 


used until experience shows a higher or lower factor to be better suited 
to the requirements of the individual. : 


Adurol 2.30. eee 5 Certinal « civ gesiieeageeeeee ee 30 
Pyrocatechin..c4 em eee 10 Amidol (2 gr. to the oz.)...... 18 
Hydrochinon (min-KBr)....... 5 Rodinal ~ ..30%1 142k 40 
Hydrochinon (max-KBr)...... 4% Ortol ace niv'a,d bd Facto ee 10 
EcOmomell 1s)... eee ee 9 Edinol *s.0 Juve se 20 
Giyein'( Carb. Soda) cy eee, 8 Metol so. 200 eee 30 
Glyeins(( Carbs: Potash) 5 eyes 12 Quinomet si <i. ss0seie ee 30 
Parammidophenol ..5..:..5.8 sae 16 


THE TECHNIQUE OF DEVELOPMENT 327 


Pyro-soda 
No With 
Potassium Bromide | -Factor Potassium bromide Factor 
ERS 4 18 Bar TN ERAS? 6 DRTORIT oo cas Wg. adn the 4 v 9 
et OZ. Oe civ kee cs oe 12 Re EOS NOT OF eas cs k vis ois v's 5 
op ins 4 a 10 Re TEDOL OZ ce coe + Wieck = so 4% 
Lt a 8 1 ET AG Sag od 8) Ara ee 4 


Pyro-acetone—about double the above. 


The factor of a combination developer containing two or more de- 
veloping agents depends upon the proportions of the various agents to 
one another. If in equal parts, the factor is simply the average of 
the factors of the two agents. But if, for instance, the developer 
contains 2 parts hydrochinon to one of metol (3 parts in all) the fac- 
tor is put down for all three parts and the sum divided by 3, or 


5+5+30_ 
3 


A combination developer containing pyro, however, does not conform 
to this rule and its factor must be determined by trial. 

Accuracy of the Factorial System.—The principle upon which the 
factorial method is based is open to, and has been made the subject of, 
some criticism. Although careful investigation has shown that there 
is not in all cases that definite and fixed relationship between the time 
of appearance and the time of development for a given gamma as as- 
sumed by the Watkins factorial method, in the vast majority of cases 
the departure from this relationship is comparatively small and with- 
out any particular significance in practical work. There are in all 
three sources of error in the factorial system. These are as follows: 


13/4. 


1. The difficulty in observing the correct time of anreaLanEe 
2. Occasional variations in the Watkins factor. 
3. Variation of the time of appearance with the degree of exposure. 


Several years ago Mr. A. Lockett conducted a number of investiga- 
tions with six different persons to determine the seriousness of the 
errors in observing the time of appearance and concluded that: 

1. What is called the personal equation in factorial development is 
of comparatively small importance, provided average care be used: 
being, in fact, much less likely to cause variation in results than with 
the old system of judging development by inspection. 

2. Although a developer with a medium factor is probably prefer- 


328 PHOTOGRAPHY 


able, there is practically no more fear of variable results with a large 
developing factor than with a small one—given reasonable care in ob- 
serving the time of appearance. : 

3. Some individuals are habitually quicker than others in observing 
the appearance of the image; but, as a rule, this variation is uniform 
and may be allowed for by an alteration in the factor. 

4. Within limits a slight error in observing the appearance of the 
image has no serious results.° 

Now that development may be conducted in bright yellow light, or 
even in white light in some cases, thanks to the introduction of satis- 
factory desensitizing agents, the difficulties in observing the appear- 
ance of the image are removed and errors from this source are practi- 
cally negligible. Desensitizing also removes another objection to the 
factorial system which was formerly of some importance. To observe 
the appearance of the image it was necessary to hold the dish close to 
the safelight and this greatly increased the danger of fog, particularly 
with color sensitive plates. All danger of fog from this source has, 
of course, been completely removed by desensitizing. 

Much has been made by critics of the factorial system of the fact 
that the Watkins factor is subject to variation with different batches of 
the same plate and developing agent. While it may be true that such 
inconstancy in the Watkins factor occurs, it is only fair to say that in 
practice it is negligible and cannot be said to constitute a serious ob- 
jection to the method. 

The variation of the time of appearance with the degree of exposure 
is the weakest point of the factorial system. It is a matter of common 
knowledge that the time of appearance of the image in development is 
greater for under exposure and less for over exposure than for normal 
exposure. Since the time of appearance is directly proportional to the 
time of development by the factorial method, the time of development 
varies with the time of appearance. Consequently under exposures 
receive longer and over exposures shorter development than do normal 
exposures. As we have already seen, the time of development required 
to reach a given stage of contrast (7) is independent of the time of ex- 
posure and hence under and over exposures should receive the same 
development as a normal exposure. Mr. Watkins advises as a means 
of lessening this source of error that several plates be developed at one 
time and the mean time of appearance taken. This is of course better 
than taking the time of appearance from a single exposure, but is at 
best only an approximation. 


9 Brit. J. Phot., 1906, 53, 464. 


yee a ae ae ee 


[* 


THE TECHNIQUE OF DEVELOPMENT 329 


With plates which have received correct, or nearly correct, exposure 
factorial development is perfectly satisfactory and in some respects the 
most desirable method of development. Where correct exposure can- 
not be ensured, thermo development is to. be preferred. 

Thermo Development.—Development for a fixed time at a certain 
temperature was indicated by Hurter and Driffield in their first paper 
“ Photochemical Investigations, etc.,” before the Liverpool Section of 
the Society of Industrial Chemistry in 1892. 

Mees and Sheppard, in a number of papers constituting in general 
a comprehensive review of the work of Hurter and Driffield in sensi- 
tometry and other allied problems relating to photographic theory, de- 
veloped the mathematical relations between the time of development 
and gamma and the effect of the velocity constant of development (k) 
and maximum contrast, or gammaimnity on the time of development. 
Their investigations, besides extending our knowledge of the physico- 
chemical factors in development, established accurate means of calcu- 
lating the time of development for any desired gamma. 

In 1903, Houdaille made the first quantitative observations on the 
rate of development at different temperatures. Two years later Fergu- 
son and Howard gave particulars of a method of calculating the times 
of development at different temperatures and a year later the former 
published the results of a more complete investigation with mathemati- 
cal formule for the calculation of the temperature coefficient. 

The fundamentals of thermo development were now established, but 
it remained for Mr. Alfred Watkins, by his carefully compiled tables 
of the developing speeds of commercial plates and times of develop- 
ment at various temperatures adapted for use with any plate, to elimi- 
nate the necessity for the individual to calculate by an exacting lab- 
oratory test the constants for his own particular case. This handicap 
removed, thermo development became exceedingly simple and was 
widely adopted both by amateurs and professionals. 

The Watkins System of Thermo Development.—The time of de- 
velopment required to reach a given stage of contrast, or gamma, de- 
pends upon: 


1. The maximum contrast obtainable (y.). 
2. The velocity constant of development (z). 
3. The temperature coefficient (T.C.). 


Methods of calculating these factors and from them the time of de- 
velopment at various temperatures for any given gamma have already 


330 PHOTOGRAPHY 


been given (pages 252 and 275-278) and from these the student can 
determine for himself the proper times of development at various tem- 
peratures with the particular plate and developer to which he is ac- 
customed. However, as many have neither the facilities nor the in- 
clination to make these calculations for themselves and yet desire to 
use the thermo method, in the following pages we will reproduce 
Watkins’ tables of commercial plates and times of development which 
are in our opinion the most comprehensive tables for time develop- 
ment yet attempted. 7 

The Watkins system takes into consideration the developing speed 
of the plate, the developer and the effect of temperature on the rate of 
development. All commercial plates are divided into eight classes ac- 
cording to the time required to reach a gamma of 0.9. ‘These classes 
are termed Very Very Quick, Very Quick, Quick, Medium Quick, 
Medium, Medium Slow, Slow, Very Slow, and designated by the 
letters VVO, VO, QO, MO, M, MS, S, and VS respectively. Instead 
of varying the time of development for each class of plates, the neces- 
sary allowance is made by altering the dilution of the developer so that 
at a normal temperature of 66° F. plates of each class require the same 


time of development. This reduces the number of scales of times of 


development to two—one for tray and one for tank development—and 

considerably simplifies the system. Six different developers are 

adapted for use with the tables which will be given shortly. These in- 

clude pyro-soda, metol-quinone, Rodinal, Azol, Citol and Certinal. 
Developing Speeds of Commercial Plates.— 


Ansco— Banner-X . . 1 9 #1 ote S 
Speedex: Pili... 55.5 see ee MS Instantaneous Iso... W553. ee M 

Barnet— Portrait Isonon,.. 4a ee M 
Ultra. Rapid’ 4.5 ae eee S Anchor .....5 V9 ie MQ 
Super Speed Ortho. .4,2¢)..5.0-0% S Commercial 2:4 25 5 Gaale eee MQ 
Studio 500.) 2240 eke ape MS Medium Iso...) 33.28 dee teen M 
Pregs}..../3.) pee ee eee VS Commercial Isonon....... PR a M 
Studio 400% \:-2) -/ erent renee MS Contrast .. 2. ein keke oa ee QO 
Studio Ortho: 200, War eee M Postal .s:c'.i35s. 256) 7 ee QO 
Red. Séal.. .. )-c5 ae de eee M Double-coated according to brand 
melt screen Ortho ...5 tie aes ae MQ Defender— —— 
Rea Diamond.’ <3. eee MS Vulcan Film 77240 eee eee S 
Special Rapid 5... Gaeeaeeeee MS Eastman Film— 
Ordinary... ©... » Sane ae eee MQ Portrait Par Speed. 5 ae eee MS 

Cramer— Super Speed 24.05, ase pee VS 
HicSneedsawt 244 ss ile eee VS Commercial; .2%. #isigeh ee ee MS 
Speeds Krome ack sia oo see S Commercial Ortho............. MS 
Crowit ate te ane oe S Panchromatic. «2.2; eae rea Ir MQ 


THE TECHNIQUE OF DEVELOPMENT 331 
PEE Verte 0 ek i MS (Me at 22 Og Bla e ce a O 
Non-Curling Speed............. S Peet LIPOIC one oo yc ie. ova eee M 
Ren ey ws. we, oR eee MS ma tid COromatics «idk bn ke QO 
Premo Speed......... cesta, ad S TO SAW Pee ss Bake Ye M 
Ce SD er MOCCH sa kcal e VS Wereatie Rapit oo. ea oh aces M 
DUETS Ga So ay ee S AS ORONO Sh teenth soa apa MQ 
RIMeOe ANGNGINACIC. 6 66h) iis es e's MS Screened Whromation.: ... i262. QO 
Pre eu UG Nah One oe MQ BS a a et aes 4 ona A's QO 
Ensign— Rapid Process Panchromatic..... VVO 
RMT Pde 8 ish Sia) x MS RVG TAINAAE Wists hey ges apd as nah QO 
Gem— Zenith Extra Sensitive Film..... VS 
SOD CORE a2) Ee S PE ALIEN ince ot IS eins i ibys RO MS 
SEE 2 SS Se, eee VS piectateesot fii. ipl). Gk weias MS 
ee base ee S PPE CSS OO TI oases als igo eye A VO 
Seon leccnromatic. 2.2... .... QO Illingworth— 
S050 (22h illite idan S sstirlio Toxtia Mast). co. Sees VS 
EOE SS Co M Pee ein de Reta Se chk ics Gaia MS 
ES Se eee VQ RAHAT AME i Geert cel at he 8 S 
RO EUS Bossi i\s ss 4 = woo M RUMOR SL Be en haha A ea oe cai MS 
Poneneomaue Uricol.. 2 ei 24}. O iid Wrnoast. is be. eee M 
RPG) ais dons as wate M PEAY UROIICP Sdie sig ais wily wecbe a > 
Pl AVET SN iis ee as bps es MO BHecial Navid... esa.k ve hak wes S 
Gevaert— PMT MCE GEN oes cs, ies ff Bs ws Ae MQ 
Se CUE Ota a 2 Say ee VS I We CELA gS maps pi coder ROME a MQ 
WA tiers Rue wpe + ow « MS Oreho Medium 6... +. 20. wears Re MQ 
PATEOPUTOMN Ge ws edges ee MS ee Eh 0 oe ne ee Ne oe Tk ae MOQ 
Pepin me. Paes. S Imperial— 
BNR me tse foc Soa 4) Cla ours MS PECISE PSPs ots ey Hiei See Baa S 
PUPEPEREMENO, 668062 ov kee es MQ PPR RRIIO Ta alee vic Geek ess oink he" 
SOUOVABE TIAGO hoc eas eee MS special! Senstivesuau.! Wve es = 
Un tte: U0tirs Oi a eee a MO Res a AE ag | Ca ee Reo a he SW MS 
Hammer— PEnCDrogi atic Faas ace ee sure O 
Se dae eS MS Pamenromiatic, Bs ica tae cle: VQ 
ED) a hee a, Coe Ue a MS SPOCIOT RADI 0.8 oe Gok ke ae aes MS 
REN BRS ee il a dials ea 4 3 O Speciotiaon Orthoy so yeaeh MS 
oiimerea Orthod has QO None titer Grihaes. pole ae 2 QO 
(eto exted Fast. 2 eco. es MS POIMESORIT biter als woseet sees M 
Sperm: ti 0 BY Giga aa @) CECE a eee ast cs 
Ortho Double-Coated.......... MS Fine Grain Ordinary... 2. ss. VVQ 
Aurora Non-Halation.......... MS Latiisea pes ey Se ae QO 
Phoeos Ostalwwie oc cs. VVQ Balmer ical ee ee oy MS 
Ilford— Lumiere & Jougla— 
OE a oy swiss VS MaRS v0 RO alah Bees ws M 
PAE T SI O S ee a > Portrajtuinstantanees 6 yim...’ M 
OC ua snl) VAS Ss ee VS (Grande Ingstantanee..). 455.4%... MQ 
CT ET fe An ah ee a VS Ortho, Yellow and Green....... O 
Special Rapid Panchromatic..... MO Ortho, Yellow and Red .o....... O 
Most Rapid Versatile........... MS ARES tie eee OE ei wig hip S05 mead a0 M 


oo2 PHOTOGRAPHY 
Panchrotiatigu. acs ek cee VQ Film... 5.2.3) coe ee M 
Inetantanee.s4 OAS eni. eee MQ Paget— 
Extra Rapide.. eieiee es tere: MOQ SOX 5 ss 0.0 ok ae ae ee 
Fils 3: oie oer eee MS ». 0.0. Gwe 
Reoroductionoiesa i 1s ok ee MQ Process Panchromatic.......... M 
Panchromatic Procede.’.....>.%5. Q S. F, Orthochromaticy= +. S 
Plavik- Pugs oes. 20 areas eee MQ Special. Rapid. 3. 3 oe oe eee 5 
COONS ears ee Se ee ee M Professional Medium........... Se 
NiiGe Speed. 625 sa seer SS) Portrait... sisasow as se S 
Sigma, Ortho.) os a.0o eee eee S Professional Extra Rapid....... 
Marion— Extra Special Rapid R.......... 
Iso Record .. 45 Syoce eee Q Ordinary Panchromatic......... 
Retord ). ee eee ee 5 Hurricane, ./72 sine eae 5 
Panchromatic: .ca0 ase eee VS Extra Special Rapid Ortho...... Ss 
Brilliant 2 .4\coiesiaee bape ee MQ Rajar— 
Po So cone dae ee eee ee MS Film, |..44 435, 33 
lristantaneouss sara. eee ea ee MS Seed— 
[sts 2. 2 eee ee eee M Graflex. 0. 5.4.55 #aeee eee VS 
Portrait 2 ¢0.ssa sae oe M Gilt Edge. 0.2.2 ee 
WB cd: Sere ee ee M Color Value. 32s yae see eee M 
Ordinary .-i55i ea ene a ee MOQ L, Ortho «<5 vin cic Paden eee M 
Stanley— 26 XX. Oo reete cst. be eee M 
BOs ob bia uace oy Cale eee M Non-Halation L Ortho......... 
Commercal, 2... os5 eee Q Tropical.) sc sa8's eee M 
Wellington— Panchromatic(: 35) -inaaereeee ~Q 
Super Ktreme. kaneis tees VS 23 0 cce weue xb Se We ee Q 
Rtremes Vac emnts eee VS Process.  vx:a.5 55s gnneiyeee ee Q 
Studio Anti-Screen............:5 de Standard— 
Préses oc ves see oes ee ee VS Extra Imperial... ..1oc3 ee ote M 
Special Extra Speedy. ..-2...-2- | Orthonon ©. 0.42.94. eee M 
Extra Speedyioy ssa oe MS Polychromeon,...i5 os pe ae M 
Speedy Portrantic, Unies ae MS Wratten & Wiasmeoviakaes 
Speedy.5 oo Civ ta eee eee S Panchromatic...........-0+.0: Q 
Anti-Screeit. 2s4.5 v.54) ore ee M Process Panchromatic.......... Q 
Iso Speedy 2.23.03 see eee M Wratten M (backed)........... QO 
Ordinary sic pa hone ee Q » 
Developers.—Thermo Pyro-Soda: 

A. Potassium metabisulphite...................05 80 gr 16 gm. 
Pyro ees tact bedew mma ee eager 160 gr 32 gm. 
Sodium -sulphite-(dry)oee 1,4. da eee I Oz. 100 gm. 
Water to make. ....555770és 6x04 pees ee 10 oz. = 1000 CC. 

B. Sodium carbonate (dry) ..s ...aces see 2 Oz. 200 gm. 
Potassium’ Bromide.) 0000", .w sss ee ee eee 40 gr. 8 gm. 
Water t6 makes oo vse as «10's 0 de ae 10 Oz 1000 cc. 


a 
[a _ oa ¢ = » 
NN ee ee en ee ee ee ee 


ee ae ee eC Um em 


Ke ee = 


THE TECHNIQUE OF DEVELOPMENT 333 


Thermo-Metol Hydrochinon (as modified by F. R. Fraprie) : 


Peer Oraenitiiy Met abDISUIphite.......cs0ce esac cae: 60 gr. 6 gm. 
Vr coach ce oe ae le il eae a0.’ -er, 3 gm. 
RIE oP eye bk eos oS cece Pele vanes gO gr. 9 gm. 
MRE ATTN co ice scydle choca deeevss ses TEU aya 1000 cc. 

OS SEO 00 2) Tee Oz, 50 gm. 
meemmentmonate Cdry) io. .is desks d seek oes 14 oz 75 gm. 
ES Se ee a ee 20.5. OZ. 1000 Cc. 


Dilution of developer. VVQ VO QO MQ M MS S VS 


PYTO-SO0G ok oa so I 14 134 244 3 4 5 634 
Metol-Hydrochinon...114 2 224 3% 44 6 Bon STO 


drams of each stock to be diluted to make a total of 3 ounces for tray or 10 
ounces for tank. 


Rodinal, Azol, Citol, . 
OPP AN sy 0% Ges 20 26 35 45 60 80 105 135 


minims solution to be made to a total of 3 ounces for tray or 9 ounces for tank. 
In metric measure 


VVQ_ VQ Q MO =2M.MS)''S> vs 


Pe Bootes, ce keys 30 41 55 73 94 125 165 210 for tray 
9 I2 16 22 28 38 50 65 for tank 
Pyro-s0dia. ws ou. es 41 55 re 94 125 165 210 280 for tray 
12 16 22 28 38 50 65 84 for tank 

Certinal, Rodinal, 
Azol, Victol..... 13 17 23 30 40 53 70 go for tray 
4.3 5.75 york 10 13 18 24 30 for tank 


The above figures are cc. and are to be diluted to a total volume of 


~ OD CC. 


Instructions.—The use of the system is simplicity itself. Deter- 


mine by reference to the table the developing speed of the plate or 


film and mix the developer as directed for that class, using water 
which has attained the same temperature as the room in which de- 
velopment is conducted. This avoids any variation in the temperature 
of the solution during development. The temperature of the developer 
having been determined, find the time of development by reference to 
the table of temperatures. In a safelight, or total darkness, flow the 
plate with the developer, or if using a tank, immerse the cage contain- 
ing the plates in the solution and start the darkroom clock. If a 
timer is not available an ordinary watch may be used. As there is no 
23 


334 PHOTOGRAPHY 


necessity whatsoever for observing the plate during development, the 
tray may be covered with a light-excluding cover and the white light 
turned on in order to observe the time at which development started. 
When the time of development has expired, turn out the white light 
and remove the plate. 


TABLE OF TIME OF DEVELOPMENT AT VARIOUS TEMPERATURES 


Degrees Degrees Cent. Time in Time for 


Fahr. to nearest half tray tank 
80 27.0 3% 12 
78 26.0 3% 13 
76 24.5 334 14 
74 23-5 4 15 
72 22.5 4% 16 
70 21 4% 17 
68 20 5 18144 
66 19 54 19 
64 18 5% 21 
62 17 644 221% 
60 16 6% 24 
58 14.5 7 26 
56 13.5 7% 28 
54 12.5 8 30 
52 11.5 8% 32 
50 10 9% a 
4 9 1o 3 
46 8 1034 40 
44 7 1 43 
42 6 1244 46 
40 4.5 1344 49 


If the first trial does not produce a negative having the proper 
amount of contrast to suit your individual case, classify the plate one 
class nearer VS for more or one class towards VVOQ for less contrast. 

The Thermo Method with Glycin.—The following formula and 
system for the use of glycin according to the Watkins thermo method 
is due to Mr. Arthur Purdon and was published in American Photog- 
raphy. 


THERMO-GLYCIN 


Stock Solution 


Water to make: 2. (oA Sas eae ee ee 500 cc. or I0 oz. 
Potassium carbonate (dry) sc...) ase ae 30 gm. or 280 er. 
sodium sulphite: (dry) 9 4.2.) sce ee -. 10 gm, or Qo gr. 


Glyein tes . of. . 3c cconvlas sane geen ee eee ee I5 gm. or 140 gr. 


~—— os ee ae ee ee ee ne Se 


THE TECHNIQUE OF DEVELOPMENT 335 


Cc. of Stock tobe Drams of Stock to be 
made up to 300cc. made up to 10 oz. for 
for Tank or 90 Tank or 3 oz. for 
Plate Glass j cc. for Tray Tray 
ON eg. ik san cn ind Sto Ve ee do 6 1% 
a eg xa a gap nk vada 7, 2 
Or 8 Bie ae en aD 10% 2% 
Do oes ole hee as ss 13% 3% 
ee ceed c cane wne bet 18: 4% 
ee eds oa ci vac dev obec ds 24 6 
re a 30 8 
I a! 4 cus ons Ve otek. Skee 40% Io 
Temperature—Time Table 
Temp. Time Time 
Degrees Minutes Minutes and Seconds 
Fahr. Tank Tray 
oh pe irre 10 2M. s0 S. 
a a 11k 3 M. 10 S. 
Dre ce cee ee 123% 3 M. 30 S. 
ree ed ee cae 1314 3 M. 50 S. 
OS Ey ee 1434 AA O12 8S, 
CS 16 4 M..35 S. 
OO 17 5 M. 
I nk cv ere neces 185% 5 M. 20 S. 
as ae ge a 20 5 M. 36 S. 
OO on tS ee oe re 21% 6 M. 


Temperature coefficient, 2.2. 


The Efficiency of Time Development.—As has been stated previ- 
ously the time of development depends upon 


I. The maximum contrast of the plate (y.). 
2. The velocity constant of development (). 
@. Lhe temperature coefficient (T.C.). 


The application of any rules found for one particular batch of a 
particular plate to a different batch of the same plate must depend on 
these factors remaining constant. As a matter of fact, however, com- — 
paratively large and unordered variations in these factors occur with 
different batches of the same plate regardless of the extreme care taken 


by the manufacturers to secure uniformity in their products. The 


maximum contrast (yo) of a plate is reasonably constant from batch 
to batch, but varying circumstances ofttimes introduce considerable 
variation, amounting in some cases to 30 or 40 per cent. The velocity 
constant of development varies considerably with different batchew: of 


336 PHOTOGRAPHY 


the same plate. This is due largely to the rate at which the plates are 
dried, which even in the elaborate systems used by manufacturers is 
subject to some variation. In addition an alteration in the character 
of the gelatine used for a batch of plates may seriously alter the factor. 
Furthermore, as has already been mentioned, the temperature coeffi- 
cient is not independent of the plate, consequently a table of times of 
development at various temperatures which is applicable to one plate 
may not be applicable to another which develops at the same rate at a 
normal temperature. The temperature coefficient, however, varies but 
slightly with different batches of the same plate. 

Therefore it appears that thermo development can only be accurately 
conducted when the values for the controlling factors (y,. and Rk) are 
known for each batch of plates. Unfortunately manufacturers have 
not yet seen their way to do this, nor, except in a few isolated cases, 
have they adopted the plan of indicating the time of development for 
each batch of plates. This has been done by a few manufacturers in 
the case of panchromatic plates, but with the vast majority of plates 
no information is given of the way, nor the extent, to which they differ 
from previous batches of the same plate. It would be a decided step 
in advance if the manufacturers could be induced to indicate for each 
batch of plates the time of development required to reach gammas of 
say 0.8, I, and 1.3 with the developers regularly advised for use with 
the plate. 

Nevertheless where such information is lacking and development is 
alike for all batches of the same plate, thermo development is a singu- 
larly uniform process which yields a surprisingly high percentage of 
satisfactory results. Such errors as may occur from variations in the 
governing factors are comparatively small and seldom sufficient to be 
of serious consequence. It is perhaps this more than anything else 
which has prevented the individual testing of each batch of plates by 
the manufacturer, who holds, and it must be admitted with a show of 
reason, that such variations as do occur are not of sufficient importance 
to warrant the labor and expense involved in the testing of each 
individual batch of plates. Nevertheless, in spite of the extreme care 
taken by manufacturers to keep their product uniform, considerable 
variations between different batches of plates occur occasionally and 
consequently laboratory testing of each batch of plates by the manu- 
facturer would be a distinct gain in scientific accuracy and thorough- 
ness, 


4 


THE TECHNIQUE OF DEVELOPMENT 


GENERAL REFERENCE WorkKS 


BLecu—Stand-Entwicklung. 
Brown—Developers and Development. 


Hust—Entwicklung der photographische Bromsilbergelatineplatten. 


Luppo-CrRAMER—Negativ Entwicklung Bei Hellem Lichte. 10922. 
ReNGER-PatzscH—Die Stand-Entwicklung. 1920. 
SEYEWETZ—Le Negatif en Photographie. 1922. 
Watxkins—Watkins’ Manual. 10918. 
Watxkins—Photography—Its Principles and Applications. 1912. 
Modern Methods of Development. Photominiature, 1309. 


1922. 


337 


CHAPTER XIV 


FIXING AND WASHING 


The Action of Sodium Thiosulphate on Silver Halide.—Only a few 
substances are capable of dissolving the silver halides and a still 
smaller number are of practical value for fixing. Of these only two 
are of sufficient importance to justify mention. These are potassium 
cyanide and sodium thiosulphate, commonly termed sodium hypo- 
sulphite or hypo, but the hyposulphite is an entirely distinct chemi- 
cal. Potassium cyanide is much too powerful for use with gelatino- 
bromide emulsion as it tends to dissolve silver and thus weaken the 
lower deposits of the negative and for this reason sodium thiosulphate, 
which is free from such action, is generally used. The use of the 
thiosulphates is due to Sir John Herschel who drew attention to their 
solvent action on the silver halides in a paper in the Edinburgh Philo- 
sophical Journal in 1819.4 

According to Abney and Meldola, the thiosulphates dissolve silver 
halide by uniting with it to form a compound of silver-disodium-thio- 
sulphate according to the reaction: ? 


1. 2AgBr-+ NaS,O, =2NaBr + Ag,S,O, 
(silver-thiosulphate), 
2. Ag,5.O; -+ Na,5,0, == Ag,S,0O,) Nano 
(silver-monosodium-thiosulphate), 
3. Ag.5.0; * Na,S,O, + Na,S,O, = Ag,S,O, * 2Na,S,0, 
(silver-disodium-thiosulphate). 
When a very small quantity of sodium thiosulphate is brought into 
contact with a considerable excess of silver halide we have the result 


1 Edinburgh Phil. Journ., vol. I, pp. 8, 396. 
2In the terms of the reacting ion the equation may be written as follows 
(Sheppard, Elliot, Sweet, J. Frank. Inst., 1923, 195, 45): 


1. Ag +S,0, = AgS,0, 
2. AgS,O, + S,0, 2Ag(S,0,), 


338 


ae a ae ee, ee oe ie ee ee 


ee ae ee eS ee 


sa =. 


a Cer ee aN es a 
- . ' 


ee ee en ee ay ee eT eee ee ee ee ee ee ee ee ae ee ee a ee 


FIXING AND WASHING 339 


indicated by equation (1). This silver thiosulphate salt (Ag,S,O,) 
is insoluble in water but soluble in sodium thiosulphate. Conse- 
quently, in the presence of an excess of sodium thiosulphate, it is 
immediately transformed into silver-monosodium-thiosulphate (Ag,- 
S,O, + Na,5S,O0,) which, like the first salt, is insoluble in water but 
soluble in sodium thiosulphate in which it is converted into silver- 
disodium-thiosulphate (Ag,S,O,:2Na,S,O0,). This double salt is 
soluble in water and may be removed from the film by washing in 
water. 

The removal of the unaltered silver halide may therefore be con- 
sidered to consist of two operations: (1) the conversion of the in- 
soluble halide into soluble double salts by sodium thiosulphate and (2) 
the removal of this double salt by washing in water. 

The Mechanism of Fixing.—The mechanism of fixing has been 
studied by Sheppard and Mees ®* and by Warwick‘ who by very dif- 
ferent experimental methods reached substantially the same conclu- 
sions. Without discussing in detail the experimental methods of 
these investigators, for which purpose the original papers should be 
consulted, we propose to deal briefly with their principal conclusions. 

The fixing bath dissolves per unit of time a constant fraction of 
the mass of silver bromide existing in the film at the origin of the in- 
terval ‘of time considered. The amount of silver bromide left in the 
negative at any time can therefore be expressed mathematically as 


4 = a(I—k)", 


where a is the original amount of silver bromide in the negative; k, 
the fraction dissolved per unit time; +, the amount remaining after 
units of time. The value of this fraction, which may be termed the 
velocity constant of fixation, depends upon the temperature and the 
concentration of the fixing bath and is independent of the amount of 
silver bromide in the film, the quality of the gelatine, or previous 
tanning of the film by formaline or similar agents but is always 
greater, under identical conditions, with silver chloride emulsions than 
with silver bromide emulsions and for the same silver halide is more 
rapid with a decrease in the size of grain. 

The simplest explanation of the known facts is that the rate of 
fixation is determined primarily by the penetration of the sodium thio- 

3 Investigations, Phot. J., 1906, 46, 235. 

4 Amer. Phot., 1917, pp. 585, 639. 


340 PHOTOGRAPHY 


sulphate through the film, the chemical action being rapid compared 
with this. 

Influence of the Concentration of Sodium Thiosulphate and Tem- 
perature on the Time of Fixation.—The investigations on the theory 
of fixation by Sheppard and Mees were concerned primarily with the 
velocity rather than the time of fixing which last is of more interest 


MINUTES 


IN 


TIME 


40° 20° 0° 4o° so” 60° 10° 
TEMPERATURE OF FIXING BATHS (°CENTIGRADE) 


Fic. 180. Influence of Temperature on Time of Fixation 


to the practical photographer. The influence of temperature and the 
concentration of the fixing bath on the time of fixing, or more exactly 
the semi-total time, or the time of clearance, was carefully studied by 
C. Welborne Piper in 1913 and the results published in the British 
Journal of the same year.® | 

The results obtained for the effect of temperature on the time of 
fixing were plotted in the form of a series of curves (Fig. 180). 
These show that the time of fixing varies approximately in inverse 
ratio to the temperature so long as small variations of temperature 
alone are considered. The curves also show that the effect of tem- 
perature varies greatly with the concentration of the fixing bath, a 
bath of 40 per cent showing less variation with a given range of tem- 
perature than those of lower or higher concentration. The effect of 


5 Brit. J. Phot., 1913, 60, 59. 


FIXING AND WASHING 341 


temperature is especially noticeable with very strong solutions. Thus 
from the curve representing a concentration of 70 per cent it appears 
that at a temperature of 20° C. several hours would be required for 
the clearance of the film and according to Piper there is doubt that 
complete fixing would ever take place under such conditions. With 
high temperatures the differences in the time of fixing for baths of 
various concentrations become less noticeable and it is probable that 
at a sufficiently high temperature the time of fixation would be the 
same for all baths regardless of concentration, since all the curves 
are of similar character and tend to meet in a point to the right of 
the graph. This is a matter difficult to prove or disprove experi- 
mentally owing to the softening of the film at the high temperatures 
involved. 

Figure 181, from Piper’s paper, gives the curves for the effect of 
varying concentrations at the same temperature, the time in minutes 


(eS eRe 

serzeeseeera/ge 
Pee er oe eel Tee | 
TENSE Ue 
NG ae 
FONSECA ae 
> (SR GRRRRRY Ae sie 
at SSS Ain 


Time in Minutes 


10% 70%. 50% 40% 0% 60% 10% 80%, 
Strength of Hypo Solutions ( Per Cent) 


Fic. 181. Influence of Concentration of Hypo on Time of Fixation 


being plotted against the concentration and the curves representing 
the results obtained for temperatures of 14, 20, 30, 40, 50, 60, and 70° 
ee es, 0, 104, 122, 140, 158° F:). 

Influence of Ammonium Chloride on the Rapidity of Fixation.— 
Ammonium thiosulphate was recommended as a fixing agent in place 


342 PHOTOGRAPHY 


of the commonly used sodium salt by Spiller in 1868. Although it is 
much more rapid in action than sodium thiosulphate, it does not equal 
the latter in general adaptability and owing to this, and to its higher 
cost, it has never been widely used. 

The investigations of C. Welborne Piper ® have well established the 
fact that for each thiosulphate the rate of fixation is more rapid at a 
certain concentration which is variable with each of the three thio- 
sulphate salts investigated. With ammonium thiosulphate and sodium 
thiosulphate the concentrations at which the maximum rapidity of fix- 
ing is secured are 15 and 4o per cent respectively. At 15 per cent, 
ammonium thiosulphate is approximately twice as fast as the sodium 
salt at 40 per cent. The rate of fixation is practically identical at a 
concentration of 33 per cent. Above this point the sodium salt is the 
more rapid while at lower concentrations the ammonium salt is the 
more rapid. 

The rapidity of fixing is considerably increased by the addition of 
ammonium chloride to the solution of sodium thiosulphate. The in- 
crease in rapidity is probably due to the partial conversion of the 
sodium thiosulphate into the corresponding ammonium salt, but ap- 


parently some undetermined factor also plays a part in the reaction, 


since the reaction is not as fast when the proportion of ammonium 
chloride is sufficient to completely convert the sodium salt to the corre- 
sponding thiosulphate as when a lesser amount is used. Thus the in- 
crease in rapidity of fixing cannot be due entirely to the conversion of 
the sodium thiosulphate into the ammonium salt. 

The effect of adding various amounts of ammonium chloride to 
solutions of sodium thiosulphate at various concentrations was care- 
fully investigated by Lumiére and Seyewetz in 19087 and by C. Wel- 
borne Piper in 1914. . The results obtained by the latter investigator 
are shown in Fig. 182. From this it will be observed that for each 
concentration of thiosulphate there is a certain definite proportion of 
ammonium chloride which produces the maximum degree of accelera- 
tion. On increasing the proportion of ammonium chloride beyond the 
optimum point, the acceleration diminishes, the rate of diminution in- 
creasing with the concentration of sodium thiosulphate. The chart 
does not show the effect of adding ammonium chloride to a bath of 
sodium thiosulphate above 40 per cent for, since no acceleration takes 
place under these conditions, the result would be of no practical value. 


€ Brit. J. Phot., 1914, 61, 193, 437, 458, 511. 
7 Bull. Soc. franc. Phot., 1908, p. 217. 


e ee tee = Sa eee ee ee ee ’ Sr eee * BS ra at, se ee ee ka So ail ¥ 
eS eT ee, ee ee ee Ee eee Cee ey. eS COME he ee eT Se PU eee | Oe, eee nm uJ u 


FIXING AND WASHING 343 


The maximum rapidity of fixing is secured by the use of a 15 per 
cent solution of sodium thiosulphate containing % of its weight of 
ammonium chloride. The following formula is suitable for a rapid 


fixing bath for use in newspaper and similar work: 


Sodium thiosulphate (“hypo”).............. to OF I50 gm. 
PeOVTy IOTICE. os dec es os cee tees ese 334 Oz. 32.6 gm. 
PENRO ee faye. hea ec ieee ees 100 0z. 1000 CC. 


The double salts of silver formed in the fixing bath containing am- 
monium chloride were shown to be less stable than those formed in a 


iaa0eanerm 


gibi ats 
Ramen meme ene te | 


NJ 


i 
is 
ie 
i 
sl 
ty 
A 
ie 
a 
B 


macs 


e 
Ef 
Et 
a 
m4 
ea 
Ey 
if 
UA 
te 
a 
az 


ie 
a 


Fic. 182. Influence of Ammonium Chloride on the Time of Fixation 


plain bath of thiosulphate by Lumiere and Seyewetz, consequently 
rapid fixing baths containing ammonium chloride are more rapidly 
exhausted than plain solutions of sodium thiosulphate and cannot be 
used for as many plates or films. 

The addition of free ammonia increases the rapidity of fixing in 
much the same way as ammonium chloride. Owing, however, to its 
disagreeable odor and its tendency to produce a type of fog known as 
“dichloric ” it is never used in practice. 

When are Plates Fixed ?—The time of fixation is not as important 
in general photographic practice as an accurate means of determining 
when fixation is complete. A rule found in many textbooks directs 


7 ae ie PHOTOGRAPHY 


that the negative be left in the fixing solution as long again as is re- 
quired for the disappearance of the opalescent coating from the back 
of the plate. Recent investigations by Lumiere and Seyewetz * have 
shown that in a fresh fixing bath this extra time is unnecessary as 
fixation is complete when the opalescent coating has completely dis- 
appeared from the back. When the fixing bath contains less than 2 
per cent of silver salts dissolved from previously fixed plates, fixing 
is still completed when the opalescent coating disappears, but when the 
amount of silver salts is in excess of 2 per cent, or 20 grams per liter 
(87.5 grains to 10 ounces),° the removal of unaltered silver salts is in- 
complete and the residual salts are not removed by prolonging the im- 


_mersion of the plate in the solution. However, if the plate is then 


transferred to a fresh bath all residual silver salts will be removed. 


The use of two fixing baths is therefore advisable unless one cares to 
make up a fresh fixing bath for each batch of negatives and discards 


| itimmediately after use. In the first case, the first bath is used nearly 


to the point of exhaustion, the negatives being transferred from this 
solution to a fresh one in which they are allowed to remain for five 
minutes. The first bath is then discarded and its place taken by the 
second bath which is in turn replaced by a freshly prepared solution. 
When the plate is allowed to remain in the first bath until completely 
cleared, then transferred to the second bath for five minutes, there is 
no danger of imperfect fixation, hence this plan is strongly recom- 
mended in preference to the somewhat haphazard methods generally 
advised. 

Exhaustion of the Fixing Bath.—While it would be well if photog- 
raphers could be induced to use a small quantity of a fresh fixing bath 
for each plate or film, discarding the bath immediately after use, this 
is seldom, if ever, done in practice, the same bath being used continu- 
ously until its slow action indicates that its fixing power is exhausted. 
A knowledge of the number of plates, films, or prints which can be 
fixed in a given volume of bath is thus of considerable practical value, 
as it enables one to determine when the bath is exhausted and prevents 
the risk of imperfect fixation owing to the use of an overworked fixing 
bath. It is evident that the number of plates which may with safety be 
fixed in any given quantity and strength of bath is dependent upon the 
amount of silver bromide which can be completely dissolved by a given 
amount of sodium thiosulphate. This matter, together with its practi- 

8 Brit. J. Phot., 1024, 71, 172. : 

9 This corresponds to approximately 240 square inches of plate surface. 


| 
- 
ree 
Ay 
4 
7 


Pee ee eee ee 


& 
PI 


5 es wee fs hia Wt 


= 
Rie ok 
— ae . ; 
Per Fe Pero Ss 


FIXING AND WASHING 345 


cal consequences, was investigated by Lumiére and Seyewetz in 1907.1° 

The result obtained by adding various amounts of silver bromide to 
equal volumes of fixing bath of three different concentrations is set 
forth in the following table: 


I. 2. ay 4. 5. 
Rhee ae Maximum au ‘ 
: oO amount of AgBr weight Ratio between 
pao he leet silver which can be AgBr 2 and the 
hypo bromide dissolved in Ratio between | necessary to weight 
ens which can 100 cc. of bath T ends form the corresponding 
be dissolved without causing compound to the salt 
in 100 cc. a subsequent Na2S203 AgeaNa2Si0c 
of bath yellowing + AgeS203 
5 per cent ee ett, 1.25 gm. 62 per cent 3.8 gm. 33 per cent 
I 46 sé 6.8 és 8 46 60 “6 ‘6 18 ie sé éé 66 
fe ‘sh ““ 20.5 ‘“ EG “c 24 c ‘ Jae “6 i ‘i “ 


The effect of adding sodium bisulphite either alone or in combina- 
tion with chrome alum was also investigated. It was found that these 
additions reduce markedly the amount of silver bromide which is com- 
pletely dissolved by a given volume as indicated in the following table: 


4. i 
Ratio between 
Weight of Calculated the maximum 
Composition Weight of AgBr which weight of weight of 
of the AgBr which can be AgBr AgBr not 
fixing can be dissolved Ratio between | necessary to giving rise to 
bath dissolved in in 100 cc. I and 2 form the stain and the 
100 cc. of of fixing bath compound weight which 
fixing bath without NaeS203 corresponds to 
* staining the + AgeS203 the salt 
negative AgeNa2SaOc 
I5 per cent 
hypo, 
1.5 per cent 
. ; y 1.65 gm. 2 r cent I1.4 gm. |I4.5 per cent 
Eadie 6.1 gm 5 gm 7 pe 4g 4.5 per c 
bisulphite, 
lye 
15: per. cent 
hypo, 
15 per cent 
sodium 
: : ; , 2,2 8 per cent 11.4 gm. 20 per cent 
bisulphite, 5-9 gm ou te 48 P 
- 0.5 per cent 
chrome 
alum 


_ These investigations indicate that, all conditions being the same, 
relatively dilute solutions are more economical in thiosulphate and that 
solutions acidified with sodium bisulphite may not be so completely ex- 


10 Bull. Soc. franc. Phot., 1907, p. 104; Phot. J., 1907, 57, 120. 


346 PHOTOGRAPHY 


hausted in practice as when bisulphite is absent. Assuming that the 
average 9/12 cm. plate contains 0.3 gram of silver bromide, 1 liter of 
15 per cent fixing bath should fix approximately 100 plates. One liter 


of fixing bath containing 1.5 per cent sodium bisulphite should, under 


the same conditions, fix about 60 plates, while if 9.5 per cent alum is 
added about 75 plates should be fixed before the bath becomes ex- 
hausted. 

Using a 25 per cent solution of thiosulphate this would correspond 
to about 15,000 square centimeters of plate or film per liter. Approxi- 
mately this works out to about gooo square inches per gallon of 25 per 
cent thiosulphate, with bisulphite, and this figure has been confirmed 
by the Eastman Research Laboratory. 

Since thiosulphate is so cheap it is the height of false economy to 
overwork the fixing bath and owing to the fact that the above figures 
will vary to a certain extent with the type of plate, since some plates 
contain more silver bromide than others, it would perhaps be well if in 
practice the above limit be reduced somewhat, to say 7500 square 
inches per gallon. To ensure the fact that this amount is not exceeded 
accurate record should be kept of the number of square inches of plate 
surface fixed. This is most conveniently done by placing a small slate 
directly over the tank carrying the fixing bath and noting thereon the 
number of plates added each time. When calculation shows that the 
maximum number of plates permissible have been fixed, the bath is 
discarded and a new one made up. If all the plates are the same size 
calculation becomes very simple. While this may seem to be unneces- 
sary trouble, reflection will show that it is based on a sound scientific 
basis and that it is far better to go to a little trouble rather than to 
lose valuable negatives from incomplete fixation. 

The Fixation of Prints——Warwick in 1917‘ and Lumiere and 
Seyewetz again in 1924 have called attention to the very short time 
required for the fixation of bromide and gaslight prints. Both in- 
vestigators have shown that fixation is a matter of only a few seconds, 
being approximately twenty to thirty seconds in 25 per cent thiosul- 
phate and only slightly greater in a fixing bath containing bisulphite. 
The rapid fixation of paper prints is doubtless due to the porous nature 
of the support which allows the reaction to proceed from both sides of 
the emulsion. 

It does not follow from the above, however, that such short periods 
of fixation are sufficient under the conditions of ordinary practice. In 


11 Amer. Phot., 1917, 11, 639. 


; 
4 
| 
P 
4 
§ 
. 


a ? Oe 


— 


ae 2 ne Be 
ae ee |’. he 


> = thet il * uA ’ z . 5. ee 7 
- a 5 ree — ai _— ain cette sh = —->. 2 2 7 m be Sve .) - Y 
ee ee aS eee ee ee re en ee ee ee eee ee, eee a ee a 


FIXING AND WASHING 347 


the above experiments the prints were separately fixed in fresh fixing 
solutions, a totally different state of affairs from that presented in 
practical work where large numbers of prints are added to the same 
solution. When dozens of prints are being treated at the same time, 
the time required for fixing is necessarily dependent upon the time 
_which the print is exposed to the action of the fixing bath. This is a 
matter of moving the prints around in the bath individually so that 
each becomes completely exposed to the solution. It is evident that 
under such conditions the time required for perfect fixation will be 
much greater than those advised by Warwick and by Lumiére and 
Seyewetz whose results were obtained with the use of individual fixing 
baths. On the other hand, the investigations show that in cases of 
rush work prints fixed individually in fresh baths for 20 to 30 seconds 
may be expected to be reasonably permanent. 

Plain Fixing Baths.—Although fixing is accomplished by thiosul- 
phate alone, plain solutions of sodium thiosulphate are not much used 
in practical photography. This is due to the fact that the bath soon 
becomes discolored from the oxidized developer carried over on the 
surface of the negatives or prints fixed in it, and these oxidized prod- 
ucts stain the negatives or prints. There is also a tendency in warm 
weather for the gelatine to swell excessively and become soft, produc- 
ing frilling and any number of other troubles either while in the fixing 
bath itself or in the washing which follows. So far as the first objec- 
tion is concerned staining of negatives or prints in a plain fixing bath 
may be largely prevented by immersing the negative or print in a weak 
bath of acid before placing in the fixing bath. Even when an acid 
fixing bath is used, the use of a weak acid bath prior to fixing prevents 
the bath from becoming discolored and lessens danger of stain. A 
weak bath of acetic acid (1 ounce 28 per cent acetic acid to 32 ounces 
of water) is to be recommended for this purpose, especially 1 in the case 
of prints or when pyro is used as a developer. 

From Piper’s investigations on the influence of the concentration of 
the bath on the rapidity of fixation (pp. 340-341) it appears that if a 
bath giving the maximum speed of fixing and the least affected by 
temperature is desired this would be attained by the use of a solution 
of approximately 40 per cent. Various other considerations, however, 
intervene to make the employment of somewhat weaker solutions de- 
sirable. The abrupt transition from a strong thiosulphate solution to 
plain water. produces a strong tension in the swollen film, which in hot 
weather gives rise to blisters and frilling and may even cause the whole 


te a re eee 
é Dis 
+% oe oo 
. i a 4 
yb f 
suey 


348 PHOTOGRAPHY 


film to leave the glass. On this account it is usual to employ a weaker 
solution of approximately 25 per cent, such as is obtained by adding 
4 ounces of crystal thiosulphate to 16 ounces of water. 

A convenient way of preparing plain fixing baths is to dissolve one 
pound of thiosulphate in about 16 ounces of warm water and when 
dissolved add cool water to 32 ounces. Every two ounces of this stock | 
solution therefore contains 1 ounce of thiosulphate. To make up 
baths of different strengths it is diluted as follows: 


[hiosulphate required % . 
for each 20 ounces 


of bath Stock solution Water 
8 OZ eooeeeeeeee eevee ereee ee ereeeees ee eevee ee ees & 16 4 c: 
6 e@eeseveevreevneeeveeeeevoeeeeeveveeevoeeeeeeeeeeereee eee e I Zz 8 . 3 
SPA Se cs wag baw bee vile te ace he 10 10 E 
Bey wie ete vive 6 ivie bre: 6 pie bl oceig ie aiet nisl oust tal siete shane tenant 8 12 : 
3 

£2 unig ata Td whereas ip ith Rim cack kg F cee ei ee 6 14 a 
Bh Ae Ao cate, wih oii alaickdi aca ean is RAR 4 16 i 


Acid Fixing Baths.—The addition of acid to the fixing bath for the 
purpose of combining the acid clearing bath frequently necessary for 
the removal of yellow oxidation stain when a plain fixing bath is used — 
with the fixing bath itself and thus avoiding a separate operation was 
advised by Lanier in 1889. If an acid is added directly to a solution 
of thiosulphate the latter is decomposed and sulphur is precipitated, 
according to the equation 


H,S,0, 2 H,5S0, +S. 


FC ee 


Lanier showed, however, that if a weak acid such as citric, acetic, 
formic or tartaric be used the precipitation of sulphur may be avoided 
by first combining the acid with a solution of sodium sulphite. When 
this is done the bath remains clear and there is no precipitation of 
sulphur unless there is an excess of acid present. 

The most convenient method of preparing an acid fixing bath is by 
the use of sodium bisulphite which may be regarded as an acid sul- 
phite, having the formula NaHSO,. The following is an excellent 
formula: | 


Se ae ee ee Tk ey ck ene ie eee 


Sodium. thiosulphate ( hypo ")’.,. <<. .5 «4s 5a 16 oz. 250 gm. 
Sodium bisuiphite so. 9 iis ene ko a SOx 47 gm. 
Water ‘to makes 7..y..04 ave Pha er ee 64 0z 1000 Cc. 


In large establishments the bisulphite may be made up as a 50 per cent 
stock solution. One part of this stock solution to each 20 parts of 
plain fixing bath will be in the correct proportion. 


FIXING AND WASHING 349 


Potassium metabisulphite may be used in place of sodium bisulphite. 
The following formula is suitable: 


MPTP IQSHIMGLC vie ccs ccc ccevsct ess csceeees 6 oz. 300 gm. 
POtusseiuin metabisulphite..........0. 6.0 cece eee YZ oz. 25 gm. 
ETNIES 5. e x's « o.6 4 od leel ee sta cals ee ee eas 20 «02. 1000 cc. 


A formula using citric acid and sodium sulphite originated by Mr. 
E. J. Wall was strongly advised by Sir William Abney. The formula 
follows : 


a RS es co be wind fastens dceess VY oz. 15.6 gm. 
RR ETRTIEINICCULOLY) ics pacaccncdss cases esr 14 oz. 15.6 gm 
Mix in I oz. (30 cc.) of water and add to 

Sodium thiosulphate...............ceeseeeeeess 4 02. 250 gm. 
eta ION d kb sis cep Se nee dels wes woe’ 16 oz. 1000 CC. 


Acid Fixing and Hardening Baths.—The third type of fixing bath, 
the acid fixing and hardening bath, is an acid bath to which an alum 
has been added to harden the gelatine and prevent softening and frill- 
ing with its attendant troubles in hot weather. The ingredients are 
generally an acid, sodium sulphite and alum and their function and 
formula are given in the following table: ?° 


Constituent Function Formula 
Sodium thiosulphate......... Fixing agent proper, dissolves 
Svar: halides it das fees woes s Na.S,O;- 5H.O 
Re eee ae Oak Se lesa es Clearing agent, promotes swell- 


ing and increases the speed of 

fixing, reduces stain and col- 

oration and regulates hardening 

ENE 0) o4 ots BAN a's is i oho eo H,SO, 
(Sulphurous or 
organic acid) 


Gress has hs piles veces Protects thiosulphate against 
decomposition by the acid. 
Anti-stain and anti-oxidant....Na.SO, 
(Sodium sulphite) 


USSG ie a rr Hardens gelatine, prevents 
. frilling and softening......... K,SO,- Al. (SO,)s, 
(Potash alum) 
Weel Cis 50,75 


(Chrome alum) 


12 The chemical theory of the acid fixing bath has been fully discussed in a 
paper by Sheppard, Elliott and Sweet of the Eastman Research Laboratory in 
the Journal of the Franklin Institute for July, 1923, 195, 45. 

24 


350 PHOTOGRAPHY 


The following formula using potassium alum is an excellent one for 


plates, films and papers: 


Sodium thiosulphate (“hypo”)... s4.05 Bee ee 16. Oz, 250 gm. 
Water to miakese. i005 os5 al ace ee 64 02. 1000 Cc. 


Dissolve separately and add to the above 


Powdered “alum. ss. 003 ia. «is 4s dees Ole i oz. 31 gm. 
Acetic acid ‘28 pér’ cent... 6 i scales eee 3 > OZ 186 cc. 
Sodium: sulphite (dry)... 5.5. 2e0s eee eee % oz. 31 gm. 
Water. to.maké.... .. 2i.5-205 0c saa ee tye gee 312 ce. 


In mixing the last solution, or hardener, it is best to use two separate 
solutions. Dissolve the alum and sulphite each in half the total 
amount of water. Then add the acid to the sulphite solution, mix the 
two and add to the solution of thiosulphate. The hardener may be 
made up as a stock solution if desired as its keeping qualities are good. 

Troubles with the Acid Fixing and Hardening Bath.—Since it 
represents a compromise between certain physico-chemical factors and 
practical conditions, the acid fixing and hardening bath is frequently 
a source of trouble owing to the fact that the exact balance between 
the various substances has not been secured when compounding the 
same. 

If the bath turns milky on standing it is due to the acid attacking 
the thiosulphate and precipitating sulphur. This may result from 
three causes: 


1. Too much acid, or too strong acid. Most formulas call for No. 8 
or 28 per cent acetic acid and not the C.P. or Glacial. 

2. Too little sulphite, bad sulphite, or high temperature of the solution. 

3. Incorrect mixing. If the method advised above is followed there 
will be no trouble on this score. 


If the milkiness disappears on standing it is due to the use of insuffi- 
cient acid, or not enough hardener to overcome the alkalinity of the 
developer brought over on the surface of the prints or negatives. 

If the bath does not harden this is due to the use of insufficient or 
impure alum, or to the fact that the bath is alkaline or neutral rather 
than acid. The hardening action of alum is due to aluminum sul- 
phate and some grades do not have the proper proportion of this sub- 
stance and accordingly must be used in greater quantity in order to 
secure equivalent action. 

Extra Hardening Baths.—In very hot climates or other exceptional 
conditions, a bath having an even greater hardening action than that 


Ritia. ; ; 
Se er 


FIXING AND WASHING 351 


above may be an advantage and in such cases the following bath, as 
worked out by Mr. J. I. Crabtree of the Eastman Research Laboratory, 
will be found very satisfactory : 


Sodium thiosulphate (“hypo”).............. ent OZ: 250 gm. 
DerueUIOOIG (OLY). oti eee ce ae ees sere y 50 gm. 
ANIMAS Ee os bs Caer dese ss news ae 2% fl. oz. 125 cc. 
TA cies co ce eon dns sce nesces 20) 02: 1000 cc. 


Although this bath has not the keeping properties of the ordinary acid 
fixing and hardening bath it will keep for at least a week at a tempera- 
ture of 100° F. 

Owing to the irritating vapors of formaline, it is well to keep the 
bath in a tank with a tight fitting cover when not in use. 

The Mechanism of Washing.—Following fixing, the next step is 
to remove the thiosulphate from the film. This is most generally 
effected by simple washing in water, although certain substances known 
as “hypo eliminators’”’ are occasionally used. 

The rate of the elimination of thiosulphate from photographic films 
of gelatine has been investigated a number of times: by Haddon and 
Grundy, Lumiere and Seyewetz in 1910, Warwick in 1917, Elsden 
the same year and by Hickman and Spencer in 1922."* 

It has been found that, in general, thiosulphate diffuses from the 
film expotentially with time as was stated by Mees and Sheppard in 
their Investigations. In other words the amount removed in a unit of 
time is proportionate to the concentration present at the beginning of 
the period. Thus if the original concentration is 10 grams and at the 
end of five minutes’ washing, one half of this, or 5 grams, is removed, 
then, if the plate is changed to an equal volume of fresh water or kept 
in the same flowing stream, in other words if the conditions present in 
the first period are duplicated, the amount removed in this second 
period will be one half of that which remains or 2.5 grams. The 
third period will remove 1.25 grams, the fourth 0.75 gram, etc. 

This law may be expressed mathematically. Thus the quantity of 
thiosulphate washed out of the film in a period of t minutes from the 
start is given by 


13Lumiére and Seyewetz, Bull. Soc. franc. Phot., 1910. Warwick, Amer. 
Phot., 1917, p. 317; Brit. J. Phot., 1917, 64, 261. Elsden, Phot. J., 1917, 57, 90; 
Brit. J. Phot., 1917, 64, 120. Hickman and Spencer, Phot. J., 1922, 62, 225. 


352 PHOTOGRAPHY 


where A is the quantity of thiosulphate originally present, k the elimi- 
nation constant for the film (50 per cent in the above example). 


Then 
dM 
z—un | 


and therefore 


Then 


which may be written 


I initial concentration in film 
k = — lo Ss ae, eee i Ty ny ae oes ea : 
t concentration at time / 


The value of k is independent of the initial concentration and may 
be obtained from 


b Concentration at time ¢; 
= ———— log | =e 

tg — ti; -© \ Concentration at time hy 

From which the time required to reach any limiting concentration 
Cy is given by: | 


I Ct 
tr ; log C, + ta. 


So much for mathematical methods which are interesting as they . 


show theoretically to what extent the thiosulphate may be reduced by 
a given amount of washing. There is some doubt, however, as to 
their value when applied to practical work. Naturally the above 
formulas may only be used when the rate of diffusion from the film 
follows the exponential law and it is by no means established that this 
is always the case. Hickman found that under certain conditions the 
rate of elimination followed the exponential law very closely but under 
other conditions contradictory results were obtained. ; 

The Efficiency of Washing Devices.—The most comprehensive 
study of the efficiency of various types of washing devices which has 
yet been made is that by Hickman and Spencer. Owing to the errors 
in estimating very small amounts of thiosulphate by the usual starch- 
iodide method and also to the fact that such tests do not necessarily 


14 Hickman and Spencer, Ibid. 


ee a Se a — 


ee eee ee ee ee ey ee Se eae es ee ee ee ee 


FIXING AND WASHING 353 


represent the concentration of thiosulphate in the film, since this may 
be higher than that of the wash water, and also to the fact that the 
thiosulphate remaining in the film may be localized in spots rather 
than distributed uniformly throughout the film, it was decided to in- 
vestigate the matter by using a colored dye which diffuses from gela- 
tine films in the same way as thiosulphate and hence would indicate 
not only the efficiency of the washing device by the time required for 
the disappearance of the dye, but would also indicate whether the 
action was uniform over the whole plate. A dye having the required 
properties was found in tartrazine, which upon test was found to dif- 
fuse from the film in the same manner as thiosulphate but much slower 
alterations in the rate of washing therefore affect plates dyed in tar- 
trazine in the same way as those containing thiosulphate, but to a 
different extent. 

Investigation of several types of washing devices by this means 
showed that all are more or less inefficient. While the water chang- 
ing properties of the washer are of importance, agitation of the water 
is equally important, for the time required for the elimination of the 
dye was not always in proportion to the water changing properties 
of the device, but on the other hand varied considerably, under con- 
ditions otherwise identical, according to the agitation of the wash 
water. Thus plates placed in an inclined trough in which a constant 
stream of water was running from the tap washed more rapidly as the 
slope of the trough was increased. Since the amount of water which 
flowed over the plates was the same in both cases, it is evident that the 
velocity of the water over the plate is a factor of considerable im- 
portance. Tank washers were found to be of varying efficiency ac- 
cording to the provisions made for the exchange of water, but none 
were found to equal in efficiency the simple inclined trough. 

A modification of the trough washer described by Windoes in 
American Photography several years ago is, in the opinion of the 
writer, one of the most satisfactory devices to be had for the washing 
of plates. The principle and construction of the apparatus are made 
clear in Fig. 183. The plates are placed film side up on each of the 
shelves and the whole rack placed under the tap. The water flows in 
a thin, fast moving stream over the surface of the plate and then over 
the edge of the shelf on to the next plate and so on down to the bot- 
tom plate. Of course the top plate will be completely washed a little 
sooner than those beneath it owing to the fact that it receives fresh 


354 PHOTOGRAPHY 


water directly from the tap while the others receive water partially 
laden with hypo from the plates above; this is of no serious conse- 
quence and the last plate will be washed much more quickly than in 
the conventional tank washer. | 


JVEGATIVE WASHER 


WHITE MAPLE 


For Bx S Pur. 


Fic. 183. Windoe’s Washing Apparatus 


For the washing of roll film in the strip the writer knows of no 
means more efficient than the Trox film washer supplied by George 
Murphy Incorporated, 57 East Ninth Street, New York City. This 
little device (Fig. 184) sends a thin spray of fresh water down both 
sides of the film which results in quick and effective elimination of 
hypo. A further development of the same principle devised by the 
writer for cut film is illustrated in Fig. 185. This apparatus is de- 
signed for use with the usual developing holders for cut films and 
is so arranged that a thin stream of water is applied to both sides of 
the film and after trickling down the surface of the film passes off by 
the drain, D. There is thus no admixture of fresh and hypo-con- 
taminated water while every part of the film is in constant contact 


with a moving stream of fresh water. A complete description of its 


construction will be found in American. Photography for 1926, p. 536. 
The Washing of Prints.—It has always been supposed that under 
similar conditions prints could be washed free from hypo in the same 


. 
DP 
3 
3 


4 
4 
a 
a 
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x 
a 


De PL ER ee ee OR ee a Ee Se ee i OE eS yee ee are ty Oe ere gee ey mes 


eet | i 


FIXING AND WASHING 350 


-or even less time than plates owing to the fact that the hypo would 


be able to diffuse from both sides. This was based upon the assump- 
tion that the diffusion of hypo from papers followed the same ex- 
ponential law as found for plates. Hickman and Spencer, however, 
have shown that such is not the case.1* While the larger part of the 
hypo is removed from the emulsion in a comparatively short time 
(as much as 90 per cent being removed by two minutes’ washing under 


f \ 
" TROX FILM WASHER | 


NTED 1,035,274 
_George Murphy, Inc. 


WW AMIGA: SS | 
as 


Fic. 184. Trox Washer for Roll Film 


certain conditions and with certain papers) a certain amount is 
tenaciously retained by the fibers of the paper support and this is 
difficult of removal. For this reason much longer times of washing 
are required for prints than for plates or films. Prints on thin papers 
should be washed at least thirty minutes in a running stream of water 
while the thicker double weight papers should receive from one to 
one and one half hours’ washing. Increasing the velocity of the water 
or the flow of water over the prints does not decrease the time of 
washing correspondingly, as in the case of plates. Provided the 
prints are kept separated the removal of the hypo retained by the 
paper base appears to be largely a matter of time and not of amount 
of water or the velocity employed. 

Unfortunately washing devices for prints are even more unsatis- 
factory than those supplied for plates. No really efficient and entirely 
satisfactory apparatus for the washing of batches of prints has yet 
been devised. The greatest difficulty arises in keeping the prints 
properly separated in order that the water may have complete access 


15 Phot. J., 1925, 65, 443 


356 PHOTOGRAPHY 


to the surface of each print. To this end it is advisable to avoid 
overloading of the washer and to separate the prints now and then 
by hand if necessary. 

Methods for Determining the Presence of Hypo.—The various sub- 
stances which indicate the presence of hypo by the formation of vari- 


—— 
SAWERAAARRARANAAANSN SSS 
NNNNNNSANNANANA 
NNNNNANANNAANA N 
NNNNANNAANANANANA 
NAN NNNA 
NNNNANANA AD \ 
NNNNNSSNANNAANA AARARAVBVBAL 
NNNNNSANNNNNANA Poe 
NNNNNNANNANANAAW 
NNNNNNNNANANNANANR 
NNNSNSNNANNAWANA N 
NNNNNNNANANANABNR 
NNNNNNANNANA NN 
NNNNANAAN N NN 


Fic. 185. Neblette’s Washer 


ous colored compounds are useful methods of determining to what 
extent the thiosulphate has been eliminated from a batch of negatives 
or prints. : 

Perhaps the most generally useful method is permanganate. Make 
up the following formula: . 


Potassium: carbonates, 2s. +s sane sce seek cee 5S yr; I gm. 
Potassium permanganate. .,c4.... <os5 see YY er. 0.1 gm. 
Distilled water to.6..,5 «ceiduces ees oe eee 10.08. 1000 CC. 


Remove the prints or negatives from the water and catch the drip- 
pings in a test tube or small graduate. Add a few drops of the 
permanganate solution and hold the graduate up against a white 
surface so as to facilitate examination. The appearance of a green- 


ish-yellow color is an indication of the presence of hypo. Very small 


ae cot he a eer 
ee ee eee et Oi 


een ial nd a 4 die Pe Aye: : a = m € ‘ eed, 
Se ee ee ee ee SE ee ee ee ee 


Pi ees Da 
See a ee 


¥ 


FIXING AND WASHING . 357 


quantities of hypo will produce a very pale bluish color rather than 
green. 

A more delicate test is secured by using starch and iodine. Powder 
a very small lump of starch about the size of a pea in three or four 
drams (10-15 cc.) of water and boil until the solution is clear. 
When cool add one drop of a tincture of iodine (a saturated solution 
of iodine in alcohol) which will produce an intense dark blue color. 
Of this solution drop two drops into two clean test tubes and fill up 
one with distilled water and the other with water from the washing 
tank. A faint blue color should be produced in the first tube while 
in the second test tube should thiosulphate be present this will dis- 
- appear, the iodide of starch becoming colorless in its presence. The 
best method of comparing the two test tubes is by placing a sheet of 
white paper below them and looking at the same through the length 
of the tube. 

In a similar way the formation of a blue streak on the back of a 
print after being brushed with a very weak solution of iodine will 
indicate the practical absence of thiosulphate. The starch in this case 
is supplied by the paper stock and if this contains thiosulphate the 
iodine streak is discharged, while in the opposite case the blue streak 
of iodide of starch remains. Only a very weak solution of iodine 
should be used for this purpose. 

The sensitivity of such tests, however, is not great and their indi- 
cations should be accepted only as an approximation, as an indication 
of the presence of hypo rather than its complete elimination. 

Hypo Eliminators.—Many attempts have been made to find a sub- 
stance which is capable of destroying hypo or converting it to some 
substance which can be quickly washed out of the gelatine film. 
Lumiére and Seyewetz in 1902 as the result of an investigation of the 
efficiency of a number of oxidizing agents as hypo eliminators recom- 
mended for this purpose ammonium persulphate, potassium percar- 
bonate and sodium peroxide, which in solution immediately hydrolizes 
into caustic soda and oxygen. This is, I believe, the first indication 
of caustic alkalis as hypo eliminators.*® 

Gaedicke in 1906 again called attention to the fact that ammonium 
thiosulphate is more quickly eliminated from gelatine films than the 
corresponding sodium compound and suggested that the negative, 
after one minute’s washing under the tap, be transferred to a tray con- 


16 Bull. Soc. franc. Phot., 1902, p. 270. 


358 PHOTOGRAPHY 


taining a 10 per cent solution of ammonium chloride, then washed in 
four changes of water.17 The danger in this procedure is the very 
rapid decomposition of the ammonium hyposulphites formed, this 
being much more rapid than the sodium compound.** | 

A. Charriou ?® published some notes on the use of sodium bicar- 
bonate but owing to the experimental methods employed his conclu- 
sions are unconvincing and there is some question as to the accuracy 
of the results obtained by him.”° 

A. E. Amor”! gives details of some investigations on the action of 
caustic soda at various concentrations and on ammonium persulphate, 
hydrogen peroxide and potassium permanganate. An 0.2 per cent 
solution of caustic soda and potassium persulphate were found to be 
the two most efficient eliminators but neither represents a decided su- 
_ periority over washing in running water provided that this is properly 
used. The infinitesimal trace of hypo adsorbed by the gelatine and 
silver image can be displaced a little more quickly by the use of a 
mildly alkaline bath, shortening the time of washing by five or ten per 
cent. There is no saving of time, however, since a minute’s longer 
washing would do as much as an eliminator bath of the same duration. 
Altogether, then, it appears that there is no advantage whatsoever in 
the use of the so-called “eliminators” of hypo. Plenty of water 
properly applied is still the secret of thorough hypo elimination. 


GENERAL REFERENCE WORKS 


Eper—Ausfihrliches Handbuch der Photographie, 1905. 
MEES AND SHEPPARD—Theory of the Photographic Process, 1907. 
VALENTA—Photographische Chemie und Chemikalienkunde, 1922. 


17 Phot. Woch., Jan. 30, 1906, p. 41. 

18 Lumiére and Seyewetz, Brit. J. Phot., 1908, 55, 417. 
19 Compt. rend., 1923, 117, 482. 

207 oP. Clerc.cS 1 PF oogtca 1D: 

21 Brit. J. Phot., 1925, 72, 18. 


Criae LER XV. 


DEFECTS IN NEGATIVES 


Their Cause and Remedy 


The Why of Defects.—The troubles of the amateur are due to non- 
observance of the simple physical and chemical laws upon which the 
operations of exposure, development and the after processes of fixing 
and washing are based. Most photographic work is conducted in a 
haphazard, totally unscientific way. Rule-of-thumb methods, while 
reasonably certain in the hands of the experienced professional, who 
works under standardized conditions, are unsuited to the variable con- 
ditions under which the average amateur works and, when we add to 
this the carelessness and want of accuracy shown by the average 
amateur, it is not surprising that he meets with a great variety of 
troubles. In dealing with photographic products, we are dealing with 
highly sensitive and complex chemical products which naturally de- 
mand careful treatment, and we must exercise accuracy and care or 
we are sure to have trouble. The importance of systematic methods 
of working cannot be too strongly emphasized. Such methods as have 
been given in preceding chapters are based upon sound scientific facts 
and, if carried out to the letter, few will be the chances of error and the 
usable percentage of work will, consequently, be very high. System, 
accuracy, careful attention to details, cleanliness, these are the things 
which every student must observe in practice if he would make a suc- 
cess of photography. 

Thin Negatives.—A negative is thin either because it has received 
insufficient exposure or has been underdeveloped. 

An under exposed negative lacks density because the action of light 
has not been sufficient to affect the number of silver grains required to 
produce a deposit of the proper opacity. Careful examination of an 
under exposed negative will also reveal an absence of detail in the 
shadows and further examination that the gradation is also false. 
Owing to the inaccuracy of the eye, this latter may not be particularly 
noticeable, but it is nevertheless present and is detrimental to the ex- 
cellence of the finished product. There is no remedy for an under 


359 


360 PHOTOGRAPHY 


exposed negative. The only thing that can be done is to make another 
negative, if possible, and give at least double the exposure. 

An undeveloped negative is thin because development has been 
stopped before the reducer has had an opportunity to reduce the num- 
ber of exposed silver grains required to produce the proper density 
and contrast. We have learned that contrast increases as develop- 
ment is prolonged, so an under developed negative lacks contrast and 
should have been developed for a longer time or in a stronger de- 
veloper. In determining whether thinness is due to under exposure or 
to under development, the important thing to observe is the shadow 
detail. If shadow detail is lacking, then under exposure is indicated, 
while lack of contrast or “ snap,’ with full shadow detail, shows under 
development. Under exposure cannot be remedied but under develop- 
ment may be corrected by intensification, which will be treated in the 
following chapter. 

Dense Negatives.—Dense negatives may be due either to over ex- 
posure or over development. In the first case, the negative is flat, ap- 
pears fogged and perhaps unsharp, while the shadows possess too much 
detail. The highlights print gray, the half tones only a shade darker 
and the shadows also print gray, the proper gradation from light to 
dark is lacking, and the effect is flat and weak. Such a negative may 
be somewhat improved by reduction in a subtractive reducer of the 
Farmer type, as described in the following chapter. 

An over developed negative is very contrasty. The highlights, or 
dense portions, are so opaque that it is impossible to print through 
them and in attempting to do this, the shadows are considerably over 
exposed and become too dark. The remedy is to reduce in a super- 
proportional reducer. 

Fog on Negatives.—Fog may be defined as a uniform deposit of 
silver extending over and either partially or wholly obliterating the 
image. It may be either general or local and due to the accidental ad- 
mission of light during the operations previous to development, to the 
use of an unsafe light in development, or from developers contaminated 
with foreign substances or used at an excessively high temperature or 
containing an excess of alkali. In fact it may be produced from any 
number of causes and for this reason its source is quite often difficult 
to ascertain. It may simplify discussion of the subject if we differ- 
entiate between local fog, general fog produced by light and chemical 
fog. 

Local Fog.—Local fog is frequently due to faulty plate holders, 


ae ee ee eee os re ee ea eel 


oe a ee 


DEFECTS IN NEGATIVES 361 


the wood having split or the joints become loose in some places so that 
light is admitted. Examination of the plate holders and a comparison 
of the location of the fog area by means of a developed negative placed 
in the holder in the position occupied during exposure will serve to 
show if this is the trouble. 

Fog sometimes results from the use of a plate holder not made for 
the particular camera on which it is being used. While the majority 
of modern cameras take what are termed standard holders there are 
some few cameras made for differently designed holders and when 
one of these is used with a camera for which it is not designed the fit is 
imperfect and light leaks in producing local fog. 

It sometimes happens, particularly with view cameras, that after 
long use the reversible back will become worn or warped so that it 
does not fit tightly to the body of the camera and light is able to reach 
the plate. The same case occurs sometimes when a new reversing 
back 1s purchased to replace an old one. 

A frequent cause of local fog with beginners is due to improper in- 
sertion of the slide of the plate holder. Beginners have the habit of 
inserting the slide by the corner but this should never be done as it 
allows the light to pass through the trap around the edges of the slide 
and produce fog. ‘The slide should be inserted squarely so that the 
entire opening is closed at once. It is well as a matter of precaution 
to keep the camera covered well with the focussing cloth when with- 
drawing or inserting the slide in order to prevent the accidental ad- 
mission of light. 

Fog may sometimes be produced by chemical emanations from the 
wood or varnish of the plate holders. ‘This is more likely to occur with 
new plate holders or when the plates are left in the holders for several 
weeks. If this is thought to be the source of the trouble, the plate 
holders should be exposed, with the slides withdrawn, to strong sun- 
shine for several days or the woodwork painted ‘with a solution of 
potassium permanganate. 

Fog is often produced in a similar way with metal plate holders as 
supplied with many foreign hand cameras. The best remedy is to 
paint the inside of the holder with a weak solution of bichloride of 
platinum. Exposure to light and air as formerly advised is also effec- 
tive in many cases. 

General Fog Due to Light.—General fog all over the plate may be 
due either to light or to chemical fog. If the edges of the plate which 
are protected by the rabbet are perfectly clear, fog is due to an opening 


362 PHOTOGRAPHY 


in the bellows or camera front or to the bellows having worn smooth 
and shiny from constant use so that it reflects light about within the 
camera. Light leakage by the camera front or bellows may be deter- 
mined by taking the camera into a dark room and placing a lighted 
electric bulb in it, using the focussing cloth to make a light-tight en- 
trance for the cable. By inserting the lamp first in one end of the 
camera and then the other and examining the same from all angles the 
leaking of light through any minute crevice may be immediately de- 
tected. If the bellows has worn smooth and shiny on the inside so 


that it is suspected of producing fog by reflecting light about in the ~ 


camera it is best replaced by a new bellows. If any wooden or metal 
parts of the camera have worn shiny and are suspected of causing 
- trouble in the same way they should receive a coat of dead black paint 
as supplied by dealers. 

If the edge of the plate or film which was protected by the rabbet is 
also fogged, the plate was either loaded or developed in an unsafe light 
unless the fog was produced by chemical reactions. A plate is very 
much more sensitive to light when dry than after it has once been 


covered by the developing solution and for this reason it is very im- 


portant that the safelight used for loading plate holders be adapted to 
the plate and that the operation of loading be conducted as quickly and 
as far away from the safelight as possible. It is far better, and not 
at all impossible after some experience, to load plates in perfect dark- 
ness and thereby avoid all danger of light fog at this stage. 


Extreme precautions should also be taken during development to 


protect the plate from the rays of the safelight except when absolutely 
necessary for examination. Once it is seen that the plate has been 
evenly covered by the developer, there is no need to examine the same 
for several minutes, or until development is judged to be nearly com- 
pleted, when the plate may be removed and given a momentary inspec- 
tion before the light. Prolonged examination of the plate before the 
safelight is responsible for many fogged negatives as well as stains 
of various kinds. 

When desensitizers are used, greater liberty may be taken with re- 
gard to light during development but the safety of the safelight used 
for loading and for immersing the plates in the desensitizer must be 
unquestionable, just as when development is conducted under ordinary 
conditions. ! 

Chemical Fog.—A certain amount of chemical fog is inevitable.but 
with proper working conditions the amount may be reduced to a point 


me 
Z 
a 
A 


ge Oe ee a ee eee me SENT 


Joe 


DEFECTS IN NEGATIVES 363 


where it is of no importance except in sensitometry.. The amount of 
chemical fog is determined by a number of factors: namely, the nature 
of the emulsion, its age and the conditions under which it has been 
stored, the nature of the developer, impurities in the developing solu- 
tion and the time and temperature of development. 

Ultra sensitive emulsions are more likely to fog chemically than 
those of lesser sensitiveness owing to their highly sensitive character 
and to the small amount of energy required to make the silver halide 


grain developable. Low speed emulsions of the type represented by 


positive emulsions and process plates are usually practically free from 
fog unless developed under unsuitable conditions. 

The age of the plate and the conditions under which the plates have 
been stored play an important part. Sensitive material should never 
be stored where it is exposed to heat, chemicals, dampness, or in a 
room where gas is burned. 

While the common developing agents differ slightly in their fogging 
propensities, none produce sufficient fog to be of serious importance 
in practical work except where improperly used. In the presence of 
an excess of alkali, or when used in a very concentrated solution, such 
active agents as metol do tend to develop fog owing to the reduction 
of unexposed silver halide. The correct proportion of alkali to be 
used under given conditions can only be determined by experiment. 

Sulphite, whether in the form of sodium sulphite, bisulphite or 
potassium metabisulphite, is added to prevent the oxidation of the de- 
veloping solution by air, the oxidized solution tending to produce fog. 
If an excess of sulphite is added a particular kind of fog, known as 
sulphite fog, is produced. The nature of sulphite fog was carefully 
investigated by Mees and Piper in 1911 * and found to be due to the 
reduction to metallic silver of the silver salt dissolved in the emulsion 
by the sulphite of the developing solution.’ 

The oxidation of certain developing agents such as metol and hy- 
drochinon exerts a powerful fogging action. The brown oxidized 
product of pyro, however, has little fogging tendency unless in very 
strong solution. In general, however, it may be said that the oxidized 
products of all developing agents tend to produce fog when sufficiently 
concentrated. 

Oxidized samples of the developing agent used in compounding solu- 
tions are responsible for fog in many cases. Stale sodium sulphite 

1 Phot. J., I9II, 51, 226; 1912, 52, 221. 

2For another form of sulphite fog see Zeit. wiss. Phot., 1913, 280. 


es eaten de Ar - 
i) 
‘ 


364 PHOTOGRAPHY 


containing sodium sulphate causes fog indirectly by not preventing 
the oxidation of the developing solution. 

Intense fog is produced by such metallic substances as copper, tin, 
and the metallic sulphites even when present in a very small quantity. 
Less than .o1 per cent of copper sulphate added to a metol-hydrochinon 
developer will produce strong fog even on positive, low-speed emul- 
sions. The action is probably due to an acceleration of the rate of 
oxidation. | 

The activity of the developing solution becomes greater with an in- 
crease in temperature and under suitable conditions this may be suffi- 
cient to enable it to attack the unexposed silver halide and produce 
fog. For this reason a temperature above 70° F. (21° C.) is espe- 
cially conducive to fog. 

As the amount of fog grows incessantly with the time of develop- 
ment, it is unwise to attempt to force development in order to secure 
greater contrast .or shadow detail as by so doing chemical fog is de- 
veloped and this being stronger in the shadows than in the higher 
densities reduces rather than increases the contrast of the negative. 

The amount of chemical fog may be reduced by the addition of a 
soluble bromide to the developing solution but it is far better to deter- 
mine if possible the source of the trouble and take appropriate steps 
to remedy the same. The use of fresh, properly compounded solu- 
tions containing the proportions of developing agent, sulphite and alkali 
advised by the manufacturer of the brand of plates employed, and 
keeping the developer free from foreign matter and at a normal tem- 
perature of 60-65° F. will reduce the danger of chemical fog to the 
minimum. 

Dichloric Fog.—Fog which appears green by reflected light and 
red by transmitted light is termed dichloric (two-colored) fog. It was 
formerly quite common, but is now comparatively rare due to improve- 
ments in the manufacture of ultra sensitive emulsions and to the use 
of the carbonates of sodium or potassium in place of ammonia as an 
alkali. 

When examined under the microscope the fog is seen to consist of 
microscopic particles of metallic silver. The size of the particles de- 
termines their. color by transmitted light, a fog which is red in color 
consisting of very small particles. | 

Dichloric fog may be formed either in the developer or the fixing 
bath. Its formation is aided by the presence of any solvent of silver 
in the developer or the fixing bath. Frequent sources in development 


2 
] 
4 
. 
‘ 
a 
; 


DEFECTS IN NEGATIVES 365 


are the presence of such silver solvents as ammonia, or hypo, and pro- 
longed development. Nowadays it most frequently occurs as the 
result of using an old or exhausted fixing bath containing an excess of 
dissolved silver and oxidized developer and a deficiency of acid. The 
presence of sulphur due to the use of too much acid so that the thio- 
sulphate is decomposed and sulphur liberated is also a frequent cause.® 

Three methods are recommended by Lumiére and Seyewetz as being 
suitable for removal of dichloric fog. They consider the method 
which follows the most generally useful.* Immerse the plate in a 
solution of potassium permanganate (14 grain to each ounce of water) 
until the fog is dissolved, then rinse and place in a 5 per cent solution 
of sodium bisulphite or potassium metabisulphite to remove the brown 
oxide of manganese which is deposited. 

Developer Stains.—While in reality a stain may be considered to be 
any deposit foreign to the photographic image which absorbs light, 
in general the word stain is usually associated with something colored. 
A stain may therefore be considered as a deposit on a negative or 
positive whose color is foreign to that of the photographic image. 
This definition thus includes colored spots, irregular colored markings 
and general stain. 

Practically all developing agents have the property of combining 
readily with oxygen especially in alkaline solution to form colored 
oxidation products the staining properties of which are very similar 
to aniline dyes. Furthermore the process of development is in itself 
an oxidizing process and this results in an additional amount of oxida- 
tion which is of course proportional to the amount of silver reduced 
in the various portions of the image. Consequently the developed 
image consists of an image of stain superimposed over one of metallic 
silver. The amount of the stain is greater with some developers than 
with others and is considerably influenced by the proportion of the 
sulphite present in the developing solution. Pyro is a developer 
which is especially prone to give a strong stain image but there is 
really no excuse for the habitual production of badly stained negatives 
when pyro is used as the degree of staining is completely under the 
control of the worker. By increasing the proportion of sulphite to 
pyro the color may be decreased almost to a neutral black while de- 

8 For a complete discussion of the causes of dichloric fog reference should be 
made to a paper by Lumiére and Seyewetz, Bull. Soc. franc. Phot., 1903, pp. 501, 
529; Phot. J., 1903, 43, 223. 

* Bull. Soc. franc. Phot., 1903, p. 324; Phot. J., 1903, 43, 226. 

25 


366 PHOTOGRAPHY 


creasing the amount of the sulphite will increase the color. Provided 
the stain is not over intense such staining may not be objectional and 
may at times be actually an advantage as it increases considerably the 
printing contrast of the image. Most developing agents form a stain 
image, though with developers like glycin, the oxidation product of 
which is readily oxidized by sulphite, the stain image is very feeble 
and practically negligible. 

Besides the stain image just considered we may have a general 
yellow, or other colored stain, which is in effect just the same as if 
the negative had been immersed in a dye solution. When uniform 
it has no harmful effect other than to increase the time of exposure 
in printing. It is produced by the use of an old and discolored de- 
veloping solution, an insufficient amount of sulphite or the use of 
impure sulphite and in many cases by the use of an old fixing bath 
which has lost its acid reaction. Since a developer is oxidized far 
more rapidly in alkaline or neutral solutions than when in an acid 
state, as the fixing bath becomes neutralized by the alkaline developer 
carried over on the surface of the negatives, this developer oxidizes 
more and more readily so that the fixing bath is converted into prac- 
tically a weak dye solution and stains the negatives immersed therein. 
It is very important that the fixing bath be kept fresh and acid in order 
to prevent stain from this source especially when a developer which 
tends to stain readily is used. The use of a stop bath of a weak acid 
between development and fixing is also advantageous. 

To remove developer stain proceed as follows: 

First harden the film by immersion for 2 to 3 minutes in a 5 per 
cent solution of formaline and wash for 5 minutes to prevent the 
gelatine from swelling and frilling in the subsequent treatment. Then 
bleach in the following: 


A. Potassium permanganate......%. -. senna 5 gm. yes se 
Water to make... -.0)ss 4020s us eae 1 fiter. "33 >on. 
B. Sodium sulphite (common salt)................ 75 gm. 24 oz. 
Sulphuric acid (Ce-P. 356 ene 15¢e: Y% Oz. 
Water to make... i... 6s sheye ay eos oka eo bh aleenete ek Rien ane 


For use take equal parts. ‘The stock solutions keep excellently but 
not when mixed and therefore the bleaching bath should be prepared 
immediately before use. 

No particles of undissolved potassium permanganate must be al- 
lowed to remain in solution A, otherwise there will be spots and 
blemishes on the negative. 


—— = os, = 


“eo Sie ee Se ee es ee ee 


DEFECTS IN NEGATIVES 367 


The bleaching is complete in about three or four minutes after 
which the brown stain of manganese oxide is removed with a 5 per 
cent solution of bisulphite. Then rinse and develop in a strong light 
in a non-staining developer such as metol-hydrochinon.* 

Local yellow stains on prints or negatives may be removed by super- 


imposing a deep yellow filter over the negative and making a positive 


either by contact or in the camera and from this making a new nega- 
tive. A panchromatic plate must of course be used and the yellow 
filter must be a contrast and not an orthochromatic filter. 

Silver Stains—The use of an old and exhausted fixing bath con- 
taining an excess of silver in solution produces what is termed silver 
stain. A silver stain may also be produced by incomplete fixation of 
the negative in a fresh bath. In both cases the stain is due to the 
incomplete removal of the light-sensitive silver halide in the fixing 
bath. This undissolved silver halide is at first colorless but is grad- 
ually changed with time and exposure to a yellow stain. Hence the 
necessity for thorough fixing. 

In the event that it is decided to try one of the methods advised for 
removal of silver stain it is well to first make the best possible posi- 
tive from the stained negative using a deep yellow filter on a panchro- 
matic plate as previously described under developer stains, since 
there are no methods of removing silver stain chemically which are 
entirely successful. The following method advised by Mr. J. I. 
Crabtree is probably as good as any: Wash the negative thoroughly 
to remove all traces of hypo which may be present in the film and 
bathe the negative in a I per cent solution of potassium cyanide. 
(Cyanide is a deadly poison and must be handled with care.) The 
cyanide will dissolve any silver thiosulphate present and some silver 
sulphide but in time it will begin to dissolve the silver image at which 
stage the negative should be removed and thoroughly washed in 
order to prevent reduction. Immersion in a weak solution of potas- 
sium permanganate followed by washing and immersion in the cyanide 
solution will often be found of service in dealing with a very old stain. 

Miscellaneous Stains.—Stain sometimes occurs when ferricyanide 
reducer is used. ‘To remove this stain immerse the plate in 


Ee ORS AE ES gS A a A Sa a 30 gr. 6 gm. 
RCE ORR iting Gs os alae le ab od elalarw 30 gr. 6 gm. 
Re oe ow eave cig bh wae s dude eae ye IO Oz. 1000 cc. 


5T am indebted to Mr. J. I. Crabtree for the above formula which is re- 
markably efficient. Brit. J. Phot., 1921, 68, 206. 


De 


368 PHOTOGRAPHY 


Printing transferred to the gelatine owing to plates having been 
wrapped in printed matter is almost impossible to remove. Try the 
following : 


Hydrochloric “atid tociics «03s bs + 24¢ ka oe 5 drops 5.2 cc. 
Waters ce Si ed cas Sessa s 2s bo a5 ea eee eer T Of. 100° CC. 


A yellowish-white opalescence which causes negatives to appear 
as if made on opal or ground glass is caused by the presence of col- 
loidal sulphur due to the use of an improperly compounded fixing 
bath containing an excess of acid or to using the fixing bath at a very 
high temperature. In both cases there is a precipitation of sulphur 
which fixes itself in the film and produces a sulphur stain. To re- 
move a sulphur stain first harden the film in a 5 per cent solution of 
formaline, wash well and immerse in a 10 per cent solution of sodium 
sulphite at a temperature of 100 to 110° F. This is a risky pro- 
cedure but is the only means of removing such stains, 

Blue stains are most often due to iron in some form, although 
amidol produces a bluish stain which may be removed by dipping the 
plate in a 10 per cent solution of sodium carbonate. In addition to 
blue stains, iron salts may produce green or yellowish-brown spots 
and whenever these appear it is very likely that iron in the water used 
for mixing solutions, or in the water used for washing, is the source 
of the trouble. Stains and spots due to the presence of iron are gen- 
erally removable by means of the bleaching solution advised for the 
removal of developer stain. Other methods advised are the use of a 
5 per cent solution of ammonia, or a 5 per cent solution of oxalic 
acid. 

A blue-green stain apparent after fixing occurs frequently when a 
chrome alum fixing bath is used at a high temperature. There is no 
known means of removing such stains. Prevention is the only cure. 

Transparent Spots.—Small microscopic spots irregular in shape 
and sometimes almost microscopic in size are due to dust. Keep the 
inside of the camera free from dust and clean plate holders now and 
then with a rag moistened with oil. Allow sufficient time for the oil 
to evaporate before again using the holders and do not use too much 
oil in the first place. The merest trace is sufficient. Dust all plates” 
carefully before placing in holders. Use a camel’s hair brush and do 
not brush too briskly, otherwise the glass will be electrified and at- 
tract dust thus making matters worse instead of better. The use 


DEFECTS IN NEGATIVES 369 


of a stiff brush will produce friction marks and only a soft camel’s 
hair brush should be used and this but lightly. 

Small transparent spots, circular in shape, are due to air in the 
water used for diluting the developer. Distilled, or at least boiled, 
water is to be preferred for all solutions but should tap water be | 
used, it is necessary that it be allowed to stand until all the air has 
escaped. This is particularly necessary when high pressure water 
mains form the source of supply. Excessive agitation of the de- 
veloper is another source. A slow, steady, rocking motion is all that 
is required and is much better than an occasional vigorous rock. 

Small, circular, transparent spots with shaded edges are due to 
air bells adhering to the plate during development and protecting the 
emulsion from development. The diffuse edge is without doubt due 
to the slow encroachment of the developing solution. These are very 
apt to occur in tank development with closed tanks. Some workers 

find that immersing the plates in water before filling the tank with 
_ developer assists in preventing such pinholes, but undoubtedly the 
surest way is to use only water from which excess air has been ex- — 
pelled by boiling and to avoid carefully any undue agitation. 

Spots of irregular shape and about the same size as those formed by 
air bells are often found distributed along one side of the plate and 
less rarely over the whole surface. They are caused by a stale de- 
veloper. 

A spot of bare glass which is uncovered by gelatine is one of the 
few defects caused by faulty manufacture of sensitive materials and 
is seldom met with when using a reliable brand of plates or film. 

Opaque or Semi-Opaque Spots.—The most common cause of small 
irregularly shaped black or dark spots is the presence of undissolved 
particles of the developing agent on the plate during development. 
Care should be taken to thoroughly dissolve every chemical in com- 
pounding developing solutions, otherwise a few particles of the de- 
veloping agent or alkali may be left and. these when brought in con- 
tact with the sensitive material in development produce dark spots 
owing to the greater rapidity of development at such spots. 

A less common cause of such spots is the presence of iron in solu- 
tions or in the water used for washing. In this case, however, the 
spots are more likely to be colored than black. 

Brown or purple spots may be caused by dry particles of developing 
agents having settled upon the dry plate. Do not mix chemicals in 


370 PHOTOGRAPHY 


the same room in which plates are developed if possible to use an- 
other room. Spots such as these may be removed occasionally by 
using one of the methods previously advised for developer stains. 
Touching the spots with nitric acid is sometimes effective but is rather 
risky. If the worker is familiar with the use of a knife. on the film 
they are best removed in this manner. 

Yellow spots, circular in shape, are due to air bells adhering to the 
plate in the fixing bath. If observed when removing the plate from 
the fixing bath they can be removed by swabbing the plate with ab- 
sorbent cotton and re-fixing. If of considerable age there is no 
means of removal other than those given under silver stain. 

Miscellaneous Troubles.—Streaks and blotches, resembling finger 
marks, brush marks, etc., are caused by old or incorrectly compounded 
developer. They are most common with hydrochinon or pyro and 
may be overcome by using a more concentrated solution. 

Cloudy or wavy appearance of the negative is due to the use of in- 
sufficient developer to cover the plate or by not rocking the tray often 
enough during development. 

A white crystalline deposit on the surface of the dry plate indicates 
very imperfect washing. Wash the plate again and make a thorough 
job of it. Immersion in a weak acetic acid bath may assist in remov- 
ing such deposits. 

Frilling or softening of the film occurs only in very hot weather or 
when there is a wide variation in the temperatures of the successive 
baths. If it is impossible to keep the developer cool, the plate may 
be immersed in formaline (10 per cent solution) before development, 
an acid fixing bath be used and care taken to keep the temperature 
of all the baths on about the same level. Acetone may with advantage 
replace the alkali in certain developers as it does not tend to soften the 
gelatine. Amidol which does not require an alkali is also very satis- 
factory. Frilling and blisters may also be. caused by using a fixing 
bath that is too strong. There is no necessity for using a fixing bath 


containing more than 30 per cent sodium thiosulphate and at such — 


concentrations there is but little danger of blisters or frilling except 
under abnormal conditions. . 

Negatives which are uneven in density due to having dried more 
. rapidly in some places than in others may frequently be improved by 
bleaching and redevelopment as already described. 


CHAPTER XVI 
REDUCTION AND INTENSIFICATION 


Part I. REDUCTION 


Reduction and the Three Classes of Reducers.—The operation by 
which some of the metallic silver forming the image is removed so as 
to secure less opacity is called reduction. All reducing agents are 
capable of converting the metallic silver into some salt which may be 
immediately dissolved away. The following table shows the different 
types of reducers and their characteristics: 


Name of Type Other Names Characteristics Examples 
Subtractive Surface All densities reduced by | Ferricyanide-hypo, 
Cutting equal amounts result- | Potassium perman- 


ing in greater contrast | ganate, Iodine- 
cyanide, Belitzski’s 


Proportional Progressive All densities reduced in} Neitz & Huse _per- 
same ratio, contrast manganate, per- 
unaltered sulphate formula 

Superpropor- Flattening The percentage reduc- | Ammonium persul- 

tional Persulphate tion is greater in the] phate, under normal 


thick parts than inthe} conditions 
thin. Results in less 
contrast 


The first comprehensive examination of a quantitative nature on 
the action of various reducers on the tones of a negative was made by 
Huse and Neitz of the Eastman Research Laboratory in 1916.1 Sensi- 
tometric strips were exposed, developed and reduced under accurately 
controlled conditions. The strips were measured before and after 
reduction in a Koenig Martens photometer, ordinary H. and D. meth- 
ods being applied to the data. The percentage of the original density 
removed by reduction from each step was then plotted against the log 
exposure for that particular density. In this way the curves of Fig. 
186 were obtained. 

Curve 1 represents a reducer of the superproportional type, repre- 
sented by ammonium persulphate ; curves II and III represent reducers 


1 Brit. J. Phot., 1916, 16, 7. 
371 


372 PHOTOGRAPHY 


of the subtractive type, curve II representing one division of this class 
of which potassium permanganate is typical and curve III another 
division of this class represented by Farmer’s ferricyanide-hypo re- 
ducer. It will be observed that this last attacks the lower densities 
more strongly than does the former. Curve IV represents a formula 


on Seed 23 plate 


Percentage of original density removed 


i 


10 
2 3 4 5B 6 7 8 9 


Fic. 186. Sensitometric Action of Different Reducers. (Nietz and Huse) 


designed by H. C. Deck for proportional reduction and Curve V a 
modification of Deck’s formula worked out by Huse and Neitz. 

Farmer’s Reducer.—A typical reducer of the subtractive type and 
one in extensive use is known as “ Farmer’s” from its originator, 
Howard Farmer, but it is also called ferricyanide-hypo reducer. It 
consists of potassium ferricyanide and “hypo.” When applied to the 
plate the silver image is oxidized by the ferricyanide and silver ferro- 
cyanide is formed which is in turn dissolved by the “ hypo” according 
to the equation 


_ 2K,Fe,(CN),. + 4Ag = 3K,Fe(CN), + Ag,Fe(CN ),. 


Since a mixture of potassium ferricyanide and “hypo” rapidly de- 
composes, it is necessary to either prepare the reducer immediately 
before using or keep two separate solutions, one containing potassium 
ferricyanide and the other hypo. : The first may be a ten per cent solu- 
tion and the latter 20 per cent. Potassium ferricyanide will keep 
fairly well in water, provided it is protected from light by being kept 
in a dark cabinet or bottle of dark green glass. 

To reduce, sufficient hypo solution (one part hypo to four of water) 
is taken to cover the negative to be reduced, to which is added a few 


. 


REDUCTION AND INTENSIFICATION 373 


drops of the potassium ferricyanide stock solution so the color of the 
solution is pale yellow—not green. Too little ferricyanide is better 
than too much, since in the latter case reduction proceeds so rapidly 
that the negative may be reduced further than desired before the action 
can be stopped. Where extreme reduction is desired the strength of 
the reducer may be increased. If at the end of five minutes reduction 
has not proceeded to the desired stage a fresh solution should be ap- 
plied. Farmer’s is a very satisfactory and convenient reducer but 
should be handled very carefully, since variations in the strength of 
the solution influence the character of the reduction—a strong solution 
tends to produce greater contrasts because it affects the shadows to a 
greater degree. : 

Belitzski’s Reducer.—This reducer is based upon the action of a 
mixture of the double oxalate of iron and potassium and “hypo” on 
the silver image. The iron salt yields its oxygen to the silver which 
forms silver oxide in a nascent condition which is at once dissolved by 
the “hypo.” The reducer keeps well in a dark place and may be used 
over and over until exhausted. In its action on the tones of the nega- 
tive it resembles Farmer’s very closely. The following is the formula : 


PeCeMBIER  PONEIC COS aIALE 4 cin cc vids ccs cs swe dace ens 150 gr. 10 gm. 
NINE MTB oes we Ge bigs baw te ve ewes 125 gr. 8 gm. 
ee oe is sens pe cs nnn s seas bse ee 7 OZ. 200 Cc. 


After completely dissolved add: 


Jealicoacid...... NE eae yo hey isc os uke ee AALS 4O gr. 2.5 gm. 


and shake until the solution turns green. Then pour off the clear 
liquid and add: 


EEL CG iy Sh ce sce kes ved obaesewsebesddas 134 02z. 50 gm. 
Instead of the potassium ferric oxalate the following may be used: 


Ferric chloride (crystal)..............c0ceeeeees 100 gr. 6.5 gm. 
POO ART ORTIALO LS cy Glen su cs ciate ses ccrcwans 190 gr. 12.5 gm. 


Mercury and Cyanide Reducer (Eder’s).—The. following reducer 
is similar to Farmer’s, but reduces more slowly, is non-staining and 
intensely poisonous: 


TEE ARIES oof 5 vey i pe le ae 8a 20 gr. 5 gm. 
BRIE SEIOC cy 45s Ged ed hoa bdo Mowe nace es 10 gr. 2.5 gm. 
BePerIO MINCHIOLICG Ss: 6.) oe ki colina ek es 10 gr. 2.5 gm. 
EOE ile Ly fiaigin sji'eculd cock w tins MA bres na's.9 10 oz. 1000 cc. 


374 PHOTOGRAPHY 


Dissolve the mercury, then the iodide and lastly the cyanide which will ° 


dissolve the red precipitate formed. On account of its intensely poi- 
sonous nature this reducer should be carefully handled and labeled 
poison. 

Iodine-Cyanide Reducer.—This is rather more energetic in its ac- 
tion on the shadows than Farmer’s and tends to clean out the lower 
densities to a greater degree without seriously affecting the higher 
densities. It is exceedingly poisonous and should be handled with 
care. It is non-staining and when used weak is a very useful reducer 
for over-developed bromide prints. 


Iodine (10 per cent sol. in potassium iodide sol.)... 30. min. 6 cc. 
Potassium cyanide (10 per cent sol. in water)...... 5 min. 1 €¢; 
Water to makesiy.. sc eee bea ek ee I oz. 100 Cc. 


Since iodine will not dissolve in water, but is readily soluble in potas- 
sium iodide, it is necessary to add about 150 grains of potassium iodide 
to just enough water to dissolve it, then add 45 grains of iodine and 
make up the solution to a total volume of one fluid ounce. 
Permanganate Reducers.—The introduction of permanganates as 
reducing agents is due to Professor Namias. The permanganates are 
strong oxidizing agents and if a solution of potassium permanganate 
containing a small amount of sulphuric acid is applied to a negative 
the silver is oxidized, forming silver sulphate, which is sufficiently 
soluble in water to be dissolved. The reaction is as follows (Namias) : 


5Ag, + 2KMnO, -++ 8H,SO, = sAg,SO, + K,SO, 
4 2MnSO, + 8H.,0O. 


Permanganate is similar in its action on the tones to Farmer’s and the 
other reducers which we have examined, but differs from them in being 
more nearly proportional in its action and not having quite the same 
“cutting’”’ effect on the lower densities. The difference in the two 
classes of reducers may be seen from the examination of Fig. 186, 
where curve II and curve III show the percentage reduction of the 
different densities for permanganate and Farmer’s respectively. 


I.: Potassium permanganate. } i)... Fale yen eee 24 gr. 50 gm. 
Water. to, make iis isis osceeee  eeee i i eee 1000 cc. 
Hl - Sulphuric acid “G2 face. os an 24 min. 50 cc. 
Water :to. mare, ook si08 pean 2 ee eee t OZ. 1000 Cc. 


For use take 1 part of A, 2 parts of B, and 64 parts of water. When 
sufficiently reduced immerse in a plain hypo solution, fresh acid fixing 


REDUCTION AND INTENSIFICATION 375 


bath, or 5 per cent solution of sodium bisulphite to remove the brown 
stain, after which wash well. 

Proportional Reducers.—-Reducers which act on all parts of the 
negative in proportion to the amount of silver present are variously 
known as proportional, true scale, and progressive, from which the 
first has been generally accepted of late as the most rational title. 
While under certain conditions ammonium persulphate may form a 
proportional reducer its action is uncertain and not to be depended 
upon but by the proper combination of potassium permanganate, which 
is a subtractive reducer, with the ammonium persulphate which is of 
the superproportional type (exactly opposite to the subtractive), a 
proportional reducer is obtained. The following formula is the one 
worked out by Huse and Neitz.” 


SoLUTION A 


POressim permanranate........0.....:...% 2.8. "gt. 0.25 gm. 
ReewereCent SUIpndric ACid....... 6. we aes TA Oz. re ee 
tee IPO tO, TAKES. . ccs cs oc ese ee eles 35 Oz. 1000 cc. 


Ammonium persulphate.................... 34 OZ. 26° ° - ern! 
0 OE ye ae a 35 Oz. 1000 CC. 


For use take one part of 4 to three of B. Do not mix until ready 
for use. The time of reduction is from one to three minutes and 
should be followed by a one per cent solution of potassium metabisul- 
phite. 

Application of Proportional Reducers.—In practice the chief pur- 
pose for which a proportional reducer is used is to reduce over dense 
negatives which are due to over development. Since over develop- 
ment increases the silver deposits proportionately the effect of reduc- 
tion in a truly proportionate reducer is to lower the gamma or in effect 
is equal to developing for a shorter length of time. 

In Fig. 187 curve I shows the characteristic curve of a plate de- 
veloped to a certain gamma. Curve II represents a gamma of unity 
(1). Now, if the negative represented by curve I is reduced in a 
proportional reducer the result will be a negative possessing the gamma 
of curve IJ. A proportional reducer is therefore the only type which 
alters density without affecting gradation. It is thus the only reducer 

2 Proportional reducers. Communication 39, Research Laboratory of East- 


man Kodak Co. British Journal of Photography, Oct. 27, 1916; Australasian 
Photo-Review, Dec. 1916. 


376 PHOT@OGRAP EY 


which may be employed without falsifying to a certain extent the 
original gradation of the negative. 

Superproportional Reducers.—Superproportional reducers are 
necessary when it is desired to reduce the contrast of a negative in 
order to make it suitable for a particular printing medium. There is 


Density 
2 


Exposure steps 


Fic. 187. Action of a Proportional Reducer on the Plate Curve 
(Nietz and Huse) 


only one reducer having a definite superproportional action and that is 
ammonium persulphate. This must be used in an acid solution and is 
rather erratic in action, sometimes acting properly and at other times 
not. Much of its irregularity is due to the presence of small amounts 
of other substances, hence in purchasing one should secure only the 


C.P. salt and this should be kept in airtight containers as it decom- 


poses in contact with air. 

Theories of Superproportional Action.—Owing to its peculiar prop- 
erty of attacking the higher densities before the lower and to its erratic 
behavior, the chemical reaction of the persulphates with the silver image 
has been the subject of much speculation, but research has not yet been 
able to explain satisfactorily the reason for its unique property of 
superproportional action. | 

A. and L. Lumiére, to whom the introduction of persulphate as a 
reducer is due, developed the following theory of its reaction: * The 
action is regarded as proceeding from the back of the negative to the 
surface in exactly the reverse method as all other operations progress, 
thus the lower densities which lie nearer to the surface are the last to 


8 Bull. Soc. franc. Phot., 1898, p. 395; Ibid., 1890, p. 226; Ibid., 1809, p. 390. 


REDUCTION AND INTENSIFICATION 377 


be attacked.* Helain® and Lauder,’ however, proved that reduction 
does not take place from the back of the film by exposing plates 
through the glass and secured the same result, while microscopical in- 
vestigation by Pigg * and by Scheffer ® shows that the action is uniform 
on all of the grains of the film and not from the back to front as 
stated in the Lumiére theory. 

In 1906 Luppo-Cramer advanced what is known as the dispersoid 
theory.® In this the behavior of persulphate is supposed to be due to 
the fact that the silver deposit is not metallic silver, as commonly sup- 
posed, but a mixture of silver and silver bromide, there being more of 
the latter in the lower densities. The superproportional action is ex- 
plained by saying that metallic silver is more soluble in persulphate 
than silver bromide—a known fact. The action of certain substances 
which are solvents of silver bromide and render the action propor- 
tional is explained by saying that the solvent removes the halide so 
that it can be more readily attacked by the persulphate. 

The catalytic theory was developed by Schuller 1° and Stenger and 
_ Heller carried on a long series of experiments to prove it.‘ This 
theory declares that the cause of the superproportional action of per- 
sulphate is due to the catalytic effect of the silver ions formed during 
the reaction of the silver and the persulphate. Since the concentration 
of these ions increases more rapidly in the higher densities than in the 
lower the action is greater on the former. Further research will be 
required, however, to definitely explain the theory of persulphate re- 
duction. 

The Practice of Persulphate Reduction.—While reduction with per- 
sulphate cannot be said to be an absolutely safe and certain process 


4 Resume Travaux Scientifiques, pp. 215, 216, 218; Brit. J. Phot., 1898 (45), 
D. 473. 

5 Theory of Persulphate Reduction, Helain, Bull. Soc. franc. Phot., 1898, 15, 
220. . 

6 Persulphate of Ammonia, H. S. Lauder, Brit. J. Phot., 1890, 46, 725. 

7“ Action of Ammonium Persulphate on the Photographic Image,” J. I. Pigg, 
Brit. J. Phot., 1903, p. 706. 

8 “ Microscopical Researches on the Effect of Persulphate and Ferricyanide 
Reducers,’ Scheffer, Brit. J. Phot., 1906, 53, 964. 

9“ Absorption Complexes in the Silver Grain as the Cause of the Persulphate 
Effect,” Phot. Korr., 1908, 45, 159. 

10“ The Theory and Practice of Reduction,” A. Schuller, Phot. Rund., 1910, 
24, 113. 

11 Z, f. Reproductions technik, 1910, 12, 162, 178 and IgII, 13, 5, 20, 34, 50, 70, 
84, 100; Zeit. wiss. Phot., 1911, 9, 73, 380. 


| Aad wy 
378 PHOTOGRAPHY : 


even with the best of care, yet by the proper observance of several im- 
portant points serious irregularities in its action will be rare. Only 
the purest persulphate should be used. Much of the commercial per- 
sulphate contains traces of iron and as Sheppard has pointed out this 
has a catalytic action.1* The amount of iron necessary to affect its 
action is on the order of I part in 1,000,000 and the limit of tolerance 
permissible is about 2 parts to 1000 of Solid persulphate. A small 
amount of iron is not a disadvantage but it is essential that the limits 
are not overstepped and also that the chemical be uniform, or the 
varying iron content of different samples of persulphate will lead to 
error. The presence of soluble chlorides, bromides, sulphates, and 
nitrates in the water used for mixing is also a source of trouble and 
many of the difficulties would disappear if the precaution of using 
distilled water was adopted. Since the characteristic action of per- 
sulphate is vitally affected by the concentration of acid present, a 
certain amount of sulphuric acid is generally added. With distilled 
water the required acidity is secured by the addition of about one 
drop C.P. sulphuric acid to each ounce of solution when freshly — 
mixed. Stock solutions of persulphate are not advisable. 

The plate should be placed in the following solution which should 
be made up just before use and distilled water only should be used: 


Ammonium persulphate............ i ee 4 gr. 8.3 gm. 
Sulphuric» acid -C; Pits.) s 2k Gene I min. Pi Oe 
Water to. make. 26. 5.h.0) sb eeewe ae eee I Oz. 1000 cc. 


The action should be watched very carefully for it becomes more 
rapid with time and the negative may be reduced further than desired 
before the action can be stopped. ‘Therefore it is better to remove 
the negative from the solution just before the reduction has reached 
the desired stage, preferably using a plate lifter to avoid contamina- 
tion with the fingers, and place in a five per cent solution of sodium 
sulphite. While refixing is not necessary it is to be advised, since it 
leaves the film amenable to further treatment. 


Part II, INTENSIFICATION 


The function of intensifiers is to increase the density and contrast 
of a negative so as to obtain better printing quality. Intensification 
may be necessary for several reasons. The negative may be simply 


12 Brit. J. Phot., 1918, 65, 314. Phot. J, America, 1918, 55, 299. 


he ae 


ee a, ee Te 


: 


REDUCTION AND INTENSIFICATION 379 


under developed due to an error in the composition of the developer, 
or the time, or temperature of the same and in such cases the intensi- 
fier continues the action of the developer, building the negative up to 
a higher degree of contrast. Owing to over exposure, or lack of con- 
trast in the subject, the negative may lack the necessary contrast and 
intensification may be desirable to supply this deficiency. 

Intensification may be effected in several ways. 

The first and most common method consists in altering the metallic 
silver grains by treatment with substances which will unite with silver 
to produce greater opacity. 

The second method consists in altering the color of the deposit so 
that it is less actinic and offers greater resistance to the passage of 
chemically active light than the original deposit. 

The third method is similar to that formerly necessary in the wet 
process for building up sufficient density and consists in adding a new 
film of silver to the old, the increased amount of silver increasing the 


_ density. 


Intensification with Mercury.—After thorough fixing and washing, 
bleach the negative in: 


PUMPER PGT OTING si vei ye tice een esse beavcees I Oz. 62 gm. 
RU PS ef ale wc psc sees aseece vas 16 oz. 1000 cc. 
After cooling add hydrochloric acid............... 30 min. Ace: 


When the negative is completely bleached through to the back of 
the plate remove and wash well in running water; if possible for at 
least twenty minutes or by giving ten five-minute soakings if washed 
inatray. It is then blackened in one of the following: 

A. Sodium sulphite 10 per cent solution 


B. An ordinary developer as Amidol, Hydrochinon, 
Ortol, Glycin, Metol-Hydrochinon, etc. 


Pr eodiine suipnattimoniate...... 6. ce. eee es 200 gr. 20 gm. 
(Schlippe’s salt) 
OE Ee a ee eae ee 20 oz. 1000 sc. 
DTA OG) 5 an x oosin cee wcsiy vin ose'e a ae 20 min. 20 ~=min. 
a eg <6 cco vk oh dars vin = woes = I oz. 30. eA CG 
E. The following ferrous oxalate developer : 
A. Potass. oxalate (neutral)........... 5 oz. Beg ~ x try, 
REEUUAL CTs fo Fas sas Vitdaebrs vo bd es 20 Oz 1000 = cc 
When cool pour off clear liquid for 
use, 
MPIOICE OL IT OM.., esac swe cc at 5 oz. 250 gm. 
Pamunric acid .C.P iss owe. Te cea 30 min. sivo"ce, 


Reet ALTN Fase oe tint nwv aie ale'sissia 20 OZ. 1000 = cc 


380 PHOTOGRAPHY 


For use take one part of B to three of A. Pour B into A and not 
vice versa. | 

The chemical reaction which takes place when the silver image is 
leached in mercuric chloride is represented by the following equa- 
tion: 


2HgCl, + 2Ag — Hg,Cl, + 2AgCl. 


The resulting chlorides of mercury and silver are transparent and 
blackening is necessary to secure printing density. With sodium sul- 
phite the reaction is as follows: 


Hg,Cl, + NaSO, + H,O — 2Hg + Na,SO, + 2HCI. 


Blackening in an alkaline developer reduces the deposit to a silver 
mercury compound whose composition is not definitely known and 
which probably varies with the developer. 

On blackening with ammonia the probable reaction is as follows: 


He,Cl, + 2NH, — NH,Hg,Cl + NH,Cl, 


When an image bleached with mercuric chloride is acted on by fer- 
rous oxalate, the image that remains consists of an amalgam of silver 
AgHg. If the process be repeated each atom takes up another atom 
of mercury and we get AgHg, and consequently greater intensifica- 
tion. The reaction would therefore be as follows: 


Hg,Cl, + 2AgCl + 4FeC,O, + 2K,C,O, 
= 2Ag + 2Hg + 2Fe,(C,0,), + 4KCL* 


Of the several methods of blackening the last is without doubt the 
most satisfactory. It gives proportionate intensification, a black de- 
posit which is permanent, and may be repeated to gain any desired 
degree of intensification. Sodium sulphite reduces the lower densi- 
ties, producing what workers call a clean result, which however is 
secured at the expense of proportional action and purity of gradation. 
There is question concerning its permanency. The objection to the 
use of developers containing sulphite is that already stated as an ob- 
jection to the use of sulphite alone but there is a further objection to 


the use of the alkali which can by itself effect a partial conversion of ~ 


the silver mercurous chloride into the carbonates or oxides. This 
possibility of two distinct reactions at one and the same time is an 


13 Chapman Jones, J. S. C. I., 1893 vol. XII, p. 983. 


REDUCTION AND INTENSIFICATION 381 


important disadvantage which tends to render the action unpropor- 
tionate and also impermanent. Sodium sulphantimoniate gives ap- 
proximate proportional intensification and with the exception of fer- 
rous oxalate is the most satisfactory of the lot. With ammonia the 
blackening. is not uniform and the reducing action in the shadows is 
very marked, the original gradation being altered to a considerable 
degree. The degree of intensification and action of the various black- 
eners on the tones of the subject will be treated at the end of the 
chapter under the Sensitometry of Intensification. 

Monckhoven’s Intensifier——The negative is bleached in mercuric 
chloride as above and blackened in the following solution of potassium 
cyanide and silver: 


Pree OOTARSI CVATIOCS, - nn dses cc ceciccseseaeccce IO er. 23 gm. 
Ree Re as aia cee coc peseuvn cs eces 10 er. 23 gm. 
ee og cca kent isessacsnceeience t -Oz. 1000 cc. 


The silver and cyanide are dissolved in separate lots of water, and the 
former solution added to the latter until a permanent precipitate is 
formed. Then allow the solution to stand fifteen minutes and filter 
after which it may be used. If the intensification is carried too far 
the plate may be reduced in “hypo.” The reaction according to 
Valenta is as follows: 


Hg,Cl, + 2AgK(CN), > 2Ag + 2He(CN), + 2KCl. 


Mercuric Iodide Intensifier.—Traces of hypo remaining in the film 
cause stains and spots with any of the above intensifiers and it is 
necessary that the greatest care be taken to thoroughly wash negatives 
before intensifying. It is a peculiar characteristic of mercuric iodide, 
and often a very valuable one, that its action is not affected by any 
traces of hypo which may remain in the film and the negative may 
be removed from the fixing bath, washed for a few minutes in water, 
and intensified at once. 


ET PECMMIPEC TT CNIOTIO€S (6.55. c cece st cens sec eeanon 175 gr. 40 gm. 
eM ae gcc Gewa edn eot ewe es 10 Oz. 1000 cc. 
Pe PORATION. 5c wirs scleine ens es ced sens eee ees I Oz. 100 gm. 
ee a op holed an obs IO Oz. 1000 cc. 


Add the larger part of the iodide to the mercury, stirring well. 
Then add the remainder of the iodide in small quantities until the so- 
lution clears. The solution changes the negative to a brown color 
which further changes to orange upon washing in water. Redevelop- 

26 


382 PHOTOGRAPHY 


ment in a non-staining developer such as amidol or metol-hydrochinon 
will render the negative less liable to ye!low in time. The chemistry 
of the reaction is as follows: ** 


2Hgl, + 2Ag = Hg,I, + 2Agl, 
Hg,I, + 2(Na,SO,) = Hg + Hgl, - (Na,SQO,).. 


Silver Intensifiers—The following formula and method for silver 
intensification is that of J. B. B. Wellington and is the revised formula 
published in 1911. 

The film should first be hardened in the following bath: 


Formaline oo... ..cess+e0ecats i evnesn cute oon nn I part 
Water 22... ceca esd seo ue vere ce sce eq tm ee sb itt nana 10 parts 


In this bath the negative should be allowed to remain for five min- 
utes, after which it should be rinsed for a few minutes and then 
placed for exactly one minute in the following bath: 


- Potassium ferricyanide...............0eceeececeeees 20 gr. 2.3 gm. 
Potassium ‘bromide... ..4.4..2is.-ae0 05 san ee 20 gr. 2.3 gm. 
Water to makes... is. 26 ss eosin ned bensa eee 20 02. 1 liter 


This bath, which should never be omitted, has the effect of pre- 
venting stains during the process of intensification. Too long an im- 
mersion in this bath causes the image to bleach, which should be 
avoided if it is desired to retain the original gradation. In the time 
prescribed there is no apparent action, but the clearing agent has done 
its work. The negative should now be rinsed for a few minutes and 
intensified in the following: 


Stock SOLUTIONS 


A. Silver: titrate. 0. ssa > oan eas se ee 800 gr. 91.2 gm. 
Distilled water to imakesvc. 035.4 5ee eee 20 Oz. 1 liter 
B. Ammonium sulphocyanide.................. 1400 gr.. 160 gm. 
ELyp6 8. a ayes we © a eee Oe 1400 gr. (160 gm. 
Water to:makei¢. t5.0050 bo i 20 Oz. I liter 


(Both solutions keep well.) 


For use take an ounce of 4 to one half ounce of B, stirring vigor- 
ously all the while the two are mixed. If stirring is omitted the solu- 
tion is apt to be turbid, whereas it should be clear. To this is added 1 
dram of a ten per cent solution of pyro solution, preserved with sul- 
phite of soda, and two drams of Io per cent solution of ammonia. 


14 Seyewetz, Le Negatif en Photographie. 


REDUCTION AND INTENSIFICATION 383 


Place negative in absolutely clean tray and pour solution over it. The 
silver begins to deposit within a minute or so and when sufficiently in- 
tensified the plate should be removed, placed in an acid fixing bath for 
a short while, and then well washed. Silver intensification is really 
physical development, silver being deposited upon the original deposit. 
The action is proportional and the results permanent and a negative 
intensified with silver may be reduced in any manner. 

Intensification with Chromium.—This process is largely due to C. 
Welborne Piper and D. J. Carnegie.1* The negative is bleached in a 
solution of potassium bichromate and hydrochloric acid and _ the 
bleached negative blackened in ordinary developer. The bleached 
image contains a chromium compound the precise formula of which is 
unknown but is thought to be CrO,. When this is treated with a de- 
veloper it is reduced and part of the chromium is left in the image in 
combination with the metallic silver. While perhaps not absolutely 
proportional in its action and thus to a certain extent falsifying grada- 
tion, the same is very slight, and as the process is easily worked and 
may be repeated over and over so that any degree of intensification 
ordinarily desirable may be had, the chromium intensifier is of great 
practical value. The degree of intensification is controlled to a certain 
extent by the amount of acid present and it is possible to vary the 
degree of intensification by altering the amount of acid, the more acid 
used the less the intensification secured, but on the whole it is more de- 
sirable to use one of the three formulas given and if the result is not 
what is desired after the first application repeat the process. The in- 
tensifier may be kept in the following stock solution from which either 
of the three bleaching baths may be compounded according to the 
degree of intensification desired: 


Pee Reet PICU FOMALC. 5... ss ses verde de seen {-02. 50 gm. 
io se chy eek ne eon ne een e's 20 OZ. 1000 cc. 
Wemeerrecuioeie Berd CPs... cee ee cee rh, Mle-Oz. 100 Cc. 
BERIT C sie, oe. owns icp eb ences tegereres IO Oz. 1000 CC. 


Baths ready for use. 


A B C Degree of intensification 
De OO ON eee ie Ss 4 Oz. 8 oz. 8 oz. A—Maximum 
Pe SOEIT os Picea secs Sidr; 20z. 8oz. $B—Medium 
PE re es ea ee 16 oz. 10 Oz. 4 02. C—Minimum 


Bleach in A, B, or C, wash until yellow stain is removed and then 
redevelop in a non-staining developer. Amidol is to be preferred, 


15 Amat. Phot., 1904, pp. 336 and 397; 1905, pp. 453 and 473. 


V ROPE aR at 


384 PHOTOGRAPHY 


especially if by any chance it is likely that the process need be repeated, 
as the change from acid to alkali is particularly hard on gelatine and by 
the use of amidol this trouble is minimized, since amidol does not re- 
quire an alkali and any tendency of the gelatine to soften and frill is 
always increased in the presence of an alkall. 

Intensification with Uranium.—If the silver image is treated with 
a ferricyanide it is reduced to a ferrocyanide, the probable reaction in 
the case of uranium ferricyanide being: 


8Ag + 4(UO,);[Fe(CN,) ], > 2Ag,Fe(CN), 
+ 3(UO,),[Fe(CN,) Jo. 


The silver image is therefore converted into a mixture of silver ferro- 
cyanide and uranyl ferrocyanide, the dark-brown or reddish color of 
which being non-actinic considerably increases the density and con- 
trast of the negative. 

Uranium is a great builder of detail and contrast and is perhaps the 
most suitable intensifier for getting the most out of an under exposed 
negative—the red deposit being able to build up to printing density all 
the detail which the exposure has been able to impress on the sensitive 
material. 

The following is a suitable formula: 


A, Uraniumenittate.. 2ewees cess ere se 100 gr. 25 gm. 
Water to. make ..:9-5 tag he ea ee 10 oz. 1000 cc. 
S.. Potassium . ferricyanide...) uta ee eee 100 gr. 25 gm. 
Water, £0 ‘make... x0. <.9.0 re be recone eee IO Oz. 1000 cc. 


For use take: A—10 parts; B—10 parts; acetic acid—2.5 parts. 

The negative must be perfectly free.from hypo or stains will re- 
sult which cannot be easily removed. When intensification is judged 
to be complete the negative should be removed and washed well in pure 
water. Hard or alkaline water cannot be used for this purpose for, 
as pointed out by Sedlaczek,!® the uranyl ferrocyanide is soluble in 
alkalis. Should the yellow stain remain after several changes of water 
its removal may be effected by means of a I0 per cent solution of 
ammonium sulphocyanide or with 


Potassium citrate.2%..03..0-00u0 ue ee 5 gm. 38 er. 
Sodium sulphate o.oo ites eee oes 25 gm. 192 gr. 
Water to make. 2x. +. oc. esun 4 toe eae 1000 cc. 16 oz. 


If for any reason it should be desirable to remove the intensification 


16 Phot. Ind., 1924, p. 234. 


ee a ee a eee wee 
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. 
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. 
| 


REDUCTION AND INTENSIFICATION 385 


altogether this may be accomplished by immersing the negative in a 
weak solution of ammonia or of sodium carbonate. If the negative is 
to be again intensified this bath should be followed by a weak bath of 
acetic acid to neutralize any traces of alkali which might remain in 
the film. 

Intensification with Lead.—Extreme intensification is secured with 
lead. Practically the only case in which such extreme contrast is re- 
quired in ordinary practice is with line subjects from poor originals. 
The general outline of the chemical reaction is the same as with 
uranium : 


2K,Fe,(CN),.-+ 4Ag + 6Pb(NO,), == Ag,Fe(CN), 
+ 3Pb,Fe(CN), + 12K NO,. 


The following formula is recommended : 


RN Ne icc np ile ats os eee cath an 400 gr. 46 gm. 
MEME RET LICVATIOG, (wi. bs sc cc open sss ose ences 600 gr. 70 gm. 
ee ee a cease tke sv tenses 3 dr. 20 cc. 
Rae Pe ee ia ss cans save n one eo wreeews 20 02. 1000 Cc. 


The stock solution will keep well in the dark. 

Bleach the negative in the above and then wash carefully in 10 per 
cent nitric acid—the acid makes the film tender—then in water and 
then darken in 


ee ETN ICG goin os ou cs ala ve de ois 0dbln ars mb dle’ ROA I Oz. 50 gm. 
RET INNO Ee Yip on Sd do ack om s vided ws e'ae nis ww bie eck 20 Oz. 1000 Cc. 


Intensification with Copper.—The copper intensifier is also only 
suited to line subjects. The reaction is as follows: 


CuSO, + 2KBr —> CuBr, + K,SO,, 
CuBr, + Ag— CuBr + AgBr. 


Applying AgNO,, 
CuBr + 2AgNO, — Cu(NO,), + Ag. Br. 


The following is a reliable formula: 


RIGOR IBLE 6 of sie 50's,0;0-4'4' + aie 5g 2 nwt lee be edie 100 gr. 230 gm. 
ey AE 6S nh bg coo is cov bole nee how das a cee I Oz. 1000 cc. 
Mremertmonitie OTOMHGE. vk. osc ce ech ave do deca ke 100 gr. 230 gm. 
BUMPER. a5 ees oa as a dete Rs Gee wa ed I Oz. 1000 cc. 


A and B are both dissolved in hot water. For use they are mixed 


386 PHOTOGRAPHY 


and the negative bleached therein after which it is washed for a minute 
or two and blackened in 


Silver nitrate;s i. 2c... me PE Ee ee 45 gr. 100 gm. 
Water: (distilled) . oo .s.. ea poten ones ae I Oz. 1000 cc. 


If too dense the negative may be reduced by the application of a 
weak solution of hypo (10 grains to the ounce) or potassium cyanide 
2 grains to the ounce. 

Intensification by Sulphiding.—A very convenient method of secur- 
ing a limited amount of intensification consists in ordinary sulphide 
toning of the image. The negative is first bleached in a bath of potas- 
sium ferricyanide and potassium bromide, then washed well and finally 
darkened in sodium sulphide. The metallic silver is thus changed to 
silver sulphide, the brown color of which is less actinic than the origi- 
nal black. Thus while the negative may actually appear less dense 
after sulphiding, its printing density has been increased by the process. 
The operation differs in no way from the toning of gaslight and bro- 
mide prints by the indirect, or redevelopment process. ‘The image is 
permanent. 

The Sensitometry of Intensification—Until quite recently no 
quantitative measurements of the character and degree of intensifica- 
tion secured with different agents had been made. This matter was 
first investigated by H. W. Bennett in 1903, by L. P. Clere in 1912,*" 
and more fully by Neitz and Huse.*® 

It is not our business here to go into the experimental methods, or 
consideration in full of the factors involved, for which the original 
paper should be consulted, but to note more particularly the character 
of the intensification secured by representative intensifiers and their 
relative efficiency. 

In the first place it will be necessary for us to notice the difference 
between visual and photographic intensification, as the two are not the 
same and we may have one without the other. If the deposit of the 
original negative is neutral and the intensified deposit also neutral, then 
any increase in visual density will be a direct measure of the photo- 
graphic effect. In most cases, however, these conditions are not ful- 
filled. Some intensifiers depend entirely upon the change of the silver 


17 Bennett, Phot. J., 1903, 43, 74. Clerc, Brit. J. Phot., 1912, 59, 215. 

18 Communication No. 58 from the Research Laboratory, Eastman Kodak 
Company, The Photographic Journal, 1918, 58, 81; Journ. Frank. Inst., March 
1918. 


REDUCTION AND INTENSIFICATION 387 


to some material having “a more non-actinic color,” as for instance 
uranium and the sulphide method. 


The authors distinguish between three general classes of intensifiers : 


I. Those giving both visual and photographic intensification, as ura- 
nium. A second class of the same giving neutral deposits, as 
mercuric bromide with amidol and chromium followed by 
amidol provided the deposit of the negative is neutral. The 
most generally useful class of intensifiers. 

2. Visual reduction but photographic intensification. Example. Re- 
development with sodium sulphide where the visual density is 
less, but the non-actinic color gives photographic intensification. 

3. Visual intensification with photographic reduction obtains only when 
intensifiers having a bleaching effect are used on negatives of 
high color. Example. Chromium-amidol.on a badly stained 
pyro negative. A special case only. 


In Fig. 188 the percentage increase in density is plotted as the 


aphic Density 


Lae) 
S 


% Increase in Photogr 


0 : A 6 8 1.0 12 
Original Density 


Fig. 188. Sensitometry of Photographic Intensification. (Nietz and Huse) 
I. Mercuric chloride+ ammonia. VI. Chromium-+ amidol. VII. Mercuric 
bromide + amidol. X. Mercuric iodide + paramidopheno!l. XII. Uranium. 
XVI. Mercuric iodide+ Schlippe’s Salt. XVIII. Cupric chloride + sodium 
stannite. 


ordinates against the original densities as abscissae. A line parallel 
with the base would thus indicate proportional intensification. No in- 


os 


388 PHOTOGRAPHY 


tensifier reaches absolute perfection in this respect although several 
approach it very closely. | 

By plotting the densities of the intensified and original plates against 
log E in the usual manner employed in sensitometry we get two char- 
acteristic curves the difference of whose gammas is a measure of the 
increase in contrast. 


photographic gamma of intensified plate y ip 
Thus a7” RES (at Varna TRY PMC SE Say oe 
photographic gamma of original plate yy op 
gives the degree of intensification. The data for a few representative 
intensifiers is given in the following table taken from the paper of Huse 


and Neitz: 


y ip 

Intensifier Blackener y op 

Mercuric : chloride... s0s0 cc vs ane ses 4k oe ammonia 1.15 
‘Potassium bichromate and HCl... s,s. s1e.5 sae amidol 1.45 
Mercuric bromide........... ER Pere te PER amidol 1.15 
Potassium ferricyanide and potassium bromide....... sodium sulphide 1.33 
Cupric chiloridé..c ceo 0 nat os es nae eee ee sodium stannite 1.03 
Permanganate and: HICl. 2... s.8 . na ee eee sodium stannite 2.05 
Mercuric. iodide. <: 04 .< 0% ca vite «awk cults ee Schlippe’s salt — 2.50 


The careful study of this and the preceding table will give the stu- 
dent much valuable information regarding the characteristics of the 
different intensifiers and their suitability for employment in particular 
cases. 

Local Reduction and Intensification.—Local reduction or intensi- 
fication is of great assistance at times in bringing out certain details in 
the shadows or in reducing the density of an over-dense highlight. If 
the negative to be reduced or intensified has been allowed to dry it 
should be first soaked for fifteen to twenty minutes in water, while if 
the negative has been handled it may be well to add to the water a 
small amount of sodium carbonate to remove any grease present on the 
film. 

It must be remembered that many intensifiers and some reducers 
(the latter, however, to a minor extent) alter not only the density but 
also the color of the deposit and this makes it hard to judge accurately 
the actual amount of reduction or intensification secured. Preference 
should therefore be given to intensifiers which do not produce a colored 
image such as chromium or mercury and ferrous oxalate. For reduc- 
tion, the iodine-cyanide reducer is well adapted but potassium perman- 
ganate or Farmer’s ferricyanide-hypo reducer may be used. 

The negative to be reduced is placed in a horizontal position on a 


eS 


: 
: 
: 


nee a ee eee. wee ee 


an Ee ale ee 


REDUCTION AND INTENSIFICATION 389 


sheet of glass where it will be well illuminated by transmitted light. 
A convenient reducing bench described by a writer in the British 
Journal of Photography is illustrated in Fig. 189. The solution should 
then be applied to the desired portions with a soft brush or with a wad 
of absorbent cotton. Use only a weak solution, otherwise the action 
may be so rapid as to get beyond control while should any of the 
strong solution be accidentally carried over on undesired areas, it will 
be impossible to prevent them from being reduced. 

Local intensification may be carried out very simply by the use of 
colored dyes. These may be applied in a very dilute state to the de- 
sired portion and allowed to dry. If too strong the negative may be 
washed in water to weaken the dye. Suitable dyes for this purpose 
are erythrosine and the Agfa preparation known as Coccine Nouvelle. 


Fic. 189. Bench for Local Reduction. (British Journal of Photography) 


Namias has recommended that the negative be immersed in a 1/1000 
solution of potassium permanganate for a few moments and the yellow 
stain removed from the portions which it is desired to darken by paint- 
ing over such portions with a solution of bisulphite of soda. 

Some workers find it advantageous to apply to the parts of the 
negative not to be acted upon by the reducing or intensifying solutions 
a water-resisting mixture which protects such portions from the action 
of the solution. The negative can then be immersed bodily in the 
solution. A varnish suitable for this purpose may be made by adding 
to benzol or chloroform a very small quantity of masticated rubber or 
pure white wax (not paraffine wax). 


GENERAL REFERENCE WORKS 


BENNETT—Intensification and Reduction, 


CHAPTER XVII 


PRINTING PROCESSES WITH SILVER SALTS 


I. PRINTING ON BROMIDE AND GASLIGHT PAPER ~ 


Development Papers.——The majority of present-day prints are 


made upon one of the many varieties of development papers. It is the 


process in general use at present and has practically pushed all others 
into the background except for particular purposes. The pictorialist 
may use one of the control processes, as gum-bichromate or oil, carbon 
or platinum, but the average amateur and professional uses develop- 
ment papers exclusively. Perhaps out of every thousand prints, only 
one is made by any other process. The reasons for the widespread 
use of development papers are not difficult to see. Since the process 
can be made more mechanical than any other, it is more economical 
and efficient for the production of prints in quantity. Owing to its 
speed, artificial light is used for exposing and we are, therefore, able to 
print at night or under any conditions of weather without being af- 
fected thereby. Commercial papers require no sensitizing or other 
operations prior to exposure, so that many of the unpleasant features 
and complexities of other processes are avoided. No other process is 
so well adapted to negatives of varying quality. Most papers are made 
in a wide variety of surfaces and in at least two colors, white and 
cream, and in at least two degrees of contrast, while generally three 
grades called hard, medium, and soft are supplied. 

There are two general divisions of development paper, bromide and 


chloro-bromide, commonly called “ gaslight.” The former is a fast 


emulsion similar to that of an ordinary dry plate, while the latter is a 
slower emulsion more suited to contact printing and having chloride of 
silver as a constituent. 

Characteristics of Development Papers.—The sensitometric char- 
acteristics of development papers were first systematically investigated 
by Mees, Jones, and Nutting of the Eastman Research Laboratories.* 
The methods of investigation are in general the same as in the sensi- 
tometry of photographic plates. Sheets of the paper to be examined 

1“ Sensitometry of Photographic Papers,” Communication No, 21, Phot. J. 
(New Series) (19014), 54, 342. 

390 


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391 


PRINTING PROCESSES WITH SILVER SALTS 


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392 PHOTOGRAPHY 


are exposed under either a sector wheel or a graduated wedge, de- 
veloped and measured in the photometer. The result when plotted in 
the usual manner gives the characteristic curve of the paper, which is 
in general form similar to the ordinary plate curve and shows practi- 
cally all of the valuable characteristics of the paper. The constants 
which serve to give a general idea of the quality of a paper are maxi- 
mum black, contrast and rendering power. These we will consider 
briefly, referring the reader for further information to the original 
paper. ; 

Maximum Black.—The maximum black is similar to the greatest 
density in the case of plates and refers to the deepest deposit which 
the paper will give. Papers which reflect above 8 per cent of the 
incident light are visibly gray-black. Papers reflecting from 6 to 3 
per cent of the incident light have strong rich blacks while papers which 
reflect less than 3 per cent have very intense blacks.2 Other things 
being equal the strongest blacks are given by glossy papers since the 
matt surface always involves scattered light. The softer the paper 
the weaker the maximum black. | 

Contrast.—As in the case of plate sensitometry the contrast of a 
paper is measured by the slope of the straight line portion and is desig- 
nated as “gamma.” However, in the case of papers the velocity of 
development is high and development is, or should be, always carried 
to the maximum (gamma infinity). The contrast of a paper, there- 
fore, refers to the value of its maximum gamma. 

Total Scale-——Another matter also having to do with contrast is what 
is known as “ total scale ”’ which refers to the number of separate tones 
which the paper can render. It is quite possible for two papers to 
have the same total scale but different values of gamma. 

Rendering Power.—By this term is meant the capacity of a paper to 
reproduce a series of exposures by a series of densities having propor- 
tional values the same as the exposure scale. Thus a paper having 
perfect rendering power when used within its total scale would give 
densities proportional to the exposure. As in plate sensitometry this 
condition is realized only within the straight line portion of the curve 
and the length of the straight line portion measured along the exposure 
axis is called the “latitude” of the paper. When gamma equals 1.0, 
then exact reproduction will be obtained within the total scale while at 
other values of gamma proportional reproduction will be obtained. 


2It is questionable if any paper reflects less than 3 per cent of the incident 
light. 


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393 


Pe ainG PROCESSES WITH SILVER SALTS 


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394 PHOTOGRAPHY 


Hard and Soft Papers.—In order to secure the best results from 
all classes of negatives, development papers are made in several grades, 
ranging from very contrasty to extreme soft. The range of contrasts 
available with bromide papers is somewhat limited as, generally speak- 
ing, only two grades, Soft and Contrasty, are made. With chloro- 
bromide or gaslight papers the range is wider and practically every 
paper of this type is supplied in at least three grades, Hard, Medium 
and Soft, while one or two have four grades. Now, on the relation 
between the scale of tones of the negative and the scale of tones of the 
printing medium depends faithful reproduction of the tones of 
the subject photographed. As we have said before, the negative is 
only a halfway step. It is the print for which we work, and it is this 
in which we wish to reproduce as accurately as possible the original 
subject. Therefore, since the relation between the scale of the nega- 
tive and the scale of the printing medium determines whether or not 
the print is to faithfully reproduce the tones of the subject, this rela- 
tion is a matter of importance to which we must give our attention. 

In Fig. 190, photograph a and its accompanying graph illustrate 
the condition resulting from the use of a long scale paper on a nega- 
tive with a shorter scale of tones. It will be observed that since the 
available scale of the printing medium is so much greater than the 
negative, the use of such paper restricts us to a scale ranging from a 
white to a grey. Thus, if the densest highlight of the negative is 
rendered as white, the deepest shadows are grey, rather than black, 
while if the exposure is adjusted so as to render the shadows black, 
the highlights of the print are degraded. In either case, the result is 
smudgy, smoky, with a washed-out appearance lacking in contrast and 
vigor. 

In the same figure, b and its accompanying graph represent the 
condition resulting from the use of a paper having approximately the 
same scale of tones as the negative. In this case, we have the densest 
highlight of the negative reproduced as white in the print and the 
deepest shadow as black, together with a full scale of intermediate 
tones, the whole resulting in a print with a pleasing gradation from 
light to dark which impresses us as being natural and proper. 

If, however, on a negative with a long scale we make use of a 
short scale paper we have the result represented in Fig. 191. In this 
case, we must loose some of the tones, for the limited range of tones 
available with the particular printing paper is insufficient for the long 


395 


PRINTING PROCESSES WITH SILVER SALTS 


DAIWBBIN 2} 0} Jodeg Bunursg oy} Jo sess oy} Sundepy ‘161 ‘OI 


SATLVOAN dO FIVOS 


MTG J0 STVOS PIV TIVAV 


XOVIG 
asasmad 


396 PHOTOGRAPHY 


scale which the negative possesses. Hence we must reproduce the 
middle tones and shadows correctly and sacrifice the highlights, or we 
must expose long enough to render the highlights properly and sacri- 
fice all detail and tone in the shadows. 

We see therefore that for faithful reproduction of the tones of the 
subject the scale of the printing medium must approximate to that 
of the negative and allow us to make the most of the full scale of 
tones available in a paper print. Now, the three negatives from 
which these prints were made represent respectively short, normal and 
long-scaled negatives produced by short, normal and prolonged de- 
-velopment. They are, in other words, what would be termed “ flat,” 
“normal” and ‘‘ hard ” or contrasty negatives. Now for a we have 
used what is termed in ordinary parlance a soft paper. The result is, 
as can readily be seen, a lack of vigor owing to the fact that the range 
of tones in the paper is greater than that of the negative. Conse- 
quently we must employ a paper having a shorter scale, or one termed 
“ medium,” “ normal,” or even “ contrast” or “hard.” In ¢ we have 
a negative with a long scale, or what would be termed a “ contrasty ” 
negative, which we have printed on a short scale paper, or in every- 
day language, a “‘ contrast ” paper, the result being excessive contrast 
- together with the loss of proper gradations in the highlights or in the 
shadows. We must therefore employ a paper with a longer scale of 
tones in order to make use of the full range of tones in the negative. 

The golden rule for selecting the proper grade of paper is, there- 
fore: Observe closely the degree of contrast in the negative. If the 
contrasts are correct, use Medium or Normal paper. If the contrasts 
are excessive, use Soft paper, while if the negative ts lacking in con- 
trast, use a Hard, Contrast or Vigorous paper. 

Exposure.—While daylight may be used for exposure, artificial 
light is preferable, owing to its greater uniformity and also to the fact 
that daylight is much too rapid for the best results, except where very 
dense negatives are involved. Practically any artificial light is usually 
suitable but electricity or gas are naturally more rapid and convenient 
in use than any of the others. Nevertheless, the common oil lamp, 
acetylene, or pocket flash lamp may be used when for any reason the 
former are not available. Magnesium ribbon also forms a very satis- 
factory illuminant, small lengths of from one half to two inches being 
used at a foot from the negative. Whatever the illuminant chosen, 
the distance between the illuminant and the printing frame should be 


. 


PemiittNG PROCESSES WITH SILVER SALTS _ 397. 


lig. 192. Printing Machines for Amateur and for Professional Use 


> Lee ne ere 
me ae Woe) 
Wea | 


398 PHOTOGRAPHY 


standardized so that it is always the same. This distance must be at 
least equal to the diagonal of the negative in order to secure even 
illumination, unless more than one light is used. Far more satisfactory 
than a printing frame, however, is one of the many types of printing 
machines which are obtainable in a wide variety of styles and prices. 
A simple machine made by the Eastman Kodak Company, especially 
for amateur use, is illustrated in Fig. 192. This printer carries a 60 
watt electric bulb and a small ruby pilot bulb. The negative is placed 
in position on the plate glass top and the paper placed over the same. 
When the platen is brought forward, the two are pressed into perfect | 
contact and at the same time the ruby light goes out and the white light 
for exposure comes on. Releasing the platen switches out the, white 
light and turns on the ruby bulb. This machine is one of many similar 
instruments which work on the same principle varying in details ac- 
cording to the requirements of the amateur, the photo-finisher or the 
professional photographer. 

Correct exposure depends upon: the density of the negative, the 
speed of the paper, the strength of the light and the distance of the 
negative from the light. Simple instruments * have been devised for 
measuring the density of the negative and from this determining the 
proper time of exposure but on the whole this is not so simple, nor so 
accurate as simply a test strip exposed and developed under actual 
working conditions, since so many varying factors alter the time of 
exposure. When once the correct exposure is found, this number to- 
gether with the paper used may be placed upon the negative envelope 
and will serve as a guide for future exposures so long as the other 
factors remain constant. 

Exposure is really determined by development and we will have 
occasion to again refer to the subject shortly. 

Developers.—There are countless numbers of formule for de- 
velopers for both bromide and chloro-bromide or gaslight papers, but 
the following two are as good as any, although it is perhaps simpler to 
follow the formula advised by the manufacturer. The first formula, 
however, may be considered as a standard developer for gaslight papers 
since it is that advised by almost every maker of such papers in 
America. The second formula is that of Wellington and Ward and is 
designed for use with bromide papers for which it is especially suitable 
but the writer has used it with various makes of gaslight paper with 
- perfect success. 


8 As, for instance, the Sanger-Shepherd Density Meter or Dawson’s Densi- 
tometer. | 


PRINTING PROCESSES WITH SILVER SALTS — 399 


STANDARD Metot-HyprRocHINON DEVELOPER 


|) er a Bt. Pr ON) taaitas-ws. ~ es rete r, 75 gm. 
IMEI EUILO NC OTY ) icc obs. ca vceee ce tees oz. 13.824 fm: 
REVELL ATA Wilicsts bovis a Gd Gos ppd vce ess 60- opr. A A ei. 
reat POnate (CTY)... seeds ancdvcrv ness 14 Oz. 12.5 gm. 
Re is gE Gob tine vn Sus oe Ah OZ. 10003 Cc, 


Potassium bromide from 5-20 grains according 
to tone desired. (.25-1 gm.) 


For convenience in compounding Mr. L. I. Snodgrass recommends 
that the developer be made up in three stock solutions: one containing 
the metol and half the quantity of sulphite, the other the hydrochinon 
with an equal amount of sulphite, and the third the sodium carbonate ; 
the three stock solutions being mixed in the proper proportions to pro- 
duce a developer adapted to the work in hand. This method has the 
added advantage that the keeping quality is better than when the alkali 
is incorporated with the developing agents. The following is the 
formula recommended by Mr. Snodgrass and the manner of dilution 
for typical soft, normal and hard-working developers: 


A B G 
LT a ee 2.5 gm. 45 gr. LaWen sees 
Sodium sulphite (dry) 18.0 gm. 34 oz. 18.0 gm. % oz. 
Hydrochinon........ ae aoe: 10.0 gm. 180 gr. eae Ay ae 
Sod. carbonate (dry) . ga oe pies ee en 36.0 gm. 1% oz. 
js 1 ae eee 500.0 cc. 200z. | 500.0 cc. 2002. | 500.0 cc. 2002. 


A B Water 
3 parts | I part I part 7 parts 
I part I part I part 3 parts 
I part 3 parts | 3 parts | 5 parts 


A 
PMormanceveroper. 2... .: ss. eee 
DoGmrreaerOeVOIODEr. 2.6 ee 


These proportions may be further varied within reasonable limits to 
secure the effect desired. If too much of stock solution B is used the 
print will have a brownish tint, while if too much carbonate (Solution 
C) is used fog will be produced. Within these limits, however, the 
developer may be varied to the degree demanded by the work in hand. 


WELLINGTON AmiIDoL DEVELOPER FOR BROMIDE PAPER 


SET SES ag 325 gr. 20 gm. 
Pe ANUUOPNENO!) ... 2. ce ce ace ce ew nnn 50 gr. a heii, 
IEG G  . 4c ne sca vas Save seadeecece 10 gr. 0.6 

EMU, icc vs doc cvs ces cs tek neesas 20 02. 500 cc. 


Amidol does not keep well in solution and the above developer should be used 
if possible the same day or at least within three days of mixing. 


400 PHOTOGRAPHY 


The Safelight.—Development should be conducted in a safe light. 
If there is any doubt concerning the safety of the light, lay a sheet of 
paper under the same in the position ordinarily occupied by the de- 
veloping tray and expose the same for a minute, then develop for a 
minute in total darkness. If there is any indication of fog, the light 
is unsafe and should be reduced in volume with a sheet of postoffice 
paper or a new safelight should be introduced. An excellent lamp for 
developing is shown in Fig. 25. For gaslight paper, the proper screen 
is the Wratten Series 00 and for bromide Series 0. Plenty of light 
may be used but it should be safe. Either of the above screens when 
used with a 16 candle power electric light will be found perfectly safe 
and will give an ideal light by which to work. 

Development.—Since practically all developing papers contain solu- 
ble bromide in the emulsion and bromide is absolutely necessary in the 
developer, there is an alteration in the characteristic curve with the 
time of development. Not only does the value of the inertia vary with 
development, but also the shape and character of the curve. Owing to 
the high velocity of development with papers, the maximum gamma of — 
development is reached very quickly, in the case of some papers within 
ten to twenty seconds, so that in practice developing papers are always 
developed to infinity. Owing to the fact that the maximum contrast 
is reached very quickly, there is a tendency, particularly in bromide 
printing where development proceeds more slowly, to remove the print 
too soon so that the maximum richness of the deposit is lost. The 
alteration which takes place in the character of the curve is graphically 
shown in the following Fig. 193 taken from the paper by Mees, Nut- 
ting and Jones of the Eastman Research Laboratory.* 

It will be observed that there is a vast difference between curves 
A and E. The latter is seriously distorted and the straight line por- 
‘tion is very short, practically non-existent, while full development 
has given a curve showing a straight line portion of considerable 
length. This condition obtains when bromide and the slow grades 
of professional gaslight papers are under developed. The condition 
is somewhat different in the case of a rapidly developing paper such 
as Velox or Cyko in which case the maximum contrast is reached 
in a very short time and times of development shorter than this show 
serious mottling and irregularity. The golden rule in developing 
both gaslight and bromide papers is then: Develop to finality or as far 

4“ The Sensitometry of Photographic Papers,” Communication No. 21, East- 
man Research Laboratory, Abridgments, vol. I, p. 68, Phot. J., 1914, 54, 342. 


. 


PRINTING PROCESSES WITH SILVER SALTS 401 


as development may be carried without producing fog. The exposure 
will then determine the darkness of the print. 

Factorial Development.—This condition is most easily secured by 
factorial development in the case of papers which develop slowly as 


[CSS a 
Bree ie) aN eer 
PT TT broke LY 

meals 


ah 


fe 
ae 
Ps 
=D 


OENSITY 


eel aeie =|) 


BEE 


& 6 li 16 18 20 2e 24 2 


Fic. 193. Effect of Time of Development upon the ene Curve of 
Paper Emulsions 


bromide and professional chloro-bromide papers and by simple time 
methods in the case of the rapidly developing gaslight papers made for 
the use of the amateur. As in the case of plate development the fac- 
torial method takes care of the variation in temperature of the devel- 
oper, and it also affords an accurate indication of the rate of develop- 
ment. Since it is customary to develop several prints, one after an- 
other, in the same volume of developer which thus becomes weak- 
ened by use the time of development grows longer and this is a factor 
difficult to determine by the method of development by inspection as 
commonly employed. Another point in favor of the factorial method 
is that, provided the proper factor is chosen, development is carried 
to infinity and the maximum quality which the paper can give is ob- 
tained. Still another point in its favor is that it makes correct ex- 
posure absolutely necéssary as a print which is over exposed will be 
too dark when developed by the factorial system, while in develop- 
ment by inspection, the print would be removed from the developer 
when the proper depth had been reached, thus resulting in under de- 
velopment and loss of print quality. 

The Proper Factor.—The proper factor seems to be entirely a 


402 PHOT OGRAPELY. 


matter of the developer and does not seem to be influenced particu- 
larly by the paper used. Thus, Kodak, Wellington, Barnet, and 
Illingsworth bromide papers have been developed in the Wellington 
formula, given above, with perfect success using a factor of 15. In 
fact, factors from 10 to 20 give practically identical results except 
that less exposure and longer development is required for the higher 
factors and in practice 15 may be chosen as a good average, since it 
is midway between the minimum and maximum useful factors. 

The proper factor for any developer may be estimated by exposing 
strips of paper under the negative for various times and developing 
the same in the developer for various times and observing accurately 
the time of appearance of the image. The time of development 
divided by the time of appearance gives the factor: 


Time of development 
= factor. 
Time of appearance 


The following is taken from tests conducted with the Eastman Amidol 
formula. 


Print No. 
I II III IV Vv 
Exposure (seconds) :s..ic02ly saicas A ie Seo eae II 15 18 27 38 
Time of appearance (seconds)............... 15 14 14 13 9 
Time of developments! 0 }> 409 2a a eee 300 | 210 | 168 | 104 54 
Factor (nearest ))\ci4 ious 2 aes ee 20 15 12 8 | 6 


Prints I, II, and III are practically identical, while IV and V show 
marked falling off in richness of blacks and in contrast. ‘The proper 
factor then is somewhere between 10 and 20, so 15 may be used as a 
standard since it is the average of the two. Time and material spent 
in determining the factor for any developer will be well repaid in the 
shape of better and more uniform print quality. 

With very rapidly developing gaslight paper the factorial method 
may be used but owing to the rapid appearance of the image in the 
developer it is rather more difficult to employ and simple development 
for the times indicated by the manufacturers in their instruction sheets 
inclosed with the paper is perhaps the best solution. Care should be 
taken, however, to keep the developer as nearly 65° F. as possible and 
to use the same for only a limited number of prints. 


Pe 
a 
q 
4 


PRINTING PROCESSES WITH SILVER SALTS 403 


The Short Stop.—While prints may be rinsed in water immediately 
following development and then placed directly in the fixing bath, in 
commercial establishments and other places where it is desirable to 
develop several prints before transferring the same to the fixing bath, 
the prints upon removal from the developer are immersed in a bath 
of acetic acid, which is termed the “short stop.” In this bath, de- 
velopment is instantly checked and the print may be left while several 
others are developed and then the entire batch transferred to the 
fixing bath at one time. In some large commercial establishments, 
it is customary to develop prints and leave them in the short stop 
until a considerable quantity have collected, when they are fixed 
together and washed. Such a “batch” may number from one to 
three hundred prints and is usually governed by the size of the fixing 
tanks and the capacity of the automatic washers. The formula for 
the acid short stop is as follows: 


REET Me eG ull kak vo Ps he paws eh vee vuleses 64 Oz. 1000 CC. 
rere cia 2h per Cent. (OOMM.) oi. cee. cece ce ees 4 OZ. 62.5) £0. 


Fixing.—Prints require to be thoroughly and completely fixed. 
Ten to fifteen minutes’ immersion in a standard acid fixing bath is 
sufficient, provided the “hypo” has complete access to the surface of 
each print. To ensure the latter condition, the prints should be con- 
stantly turned over and over so that the hypo may be able to reach 
each and every print. Merely leaving the prints immersed in a suf- 
ficient quantity of fresh acid fixing bath of proper strength for an 
indefinite time is not fixing and is to be heartily condemned. The 
golden rule for perfect fixation of prints may be stated as follows: 
Use a fresh acid fixing bath and keep the prints in motion for the: 
entire time of fixation, which should require at least fifteen minutes. 
In some commercial establishments, where large numbers of prints 
are handled in each batch, two fixing baths are used, the prints being 
fixed in one for ten to fifteen minutes and then transferred to the 
second for a similar length of time. This is a capital plan and is one 
which might well be adopted by every amateur finisher. Attention 
might well be called to the fact that the fixing bath should be acid; 
otherwise, the developer carried over upon the surface of the prints 
will soon cause it to discolor. Careful draining of the prints as they 
are removed from the developer and the use of an acid short stop be- 
tween development and fixing will do much towards keeping the fix- 


404 PHOTOGRAPHY 


ing bath clean. There is more danger of overworking the fixing bath 
with prints than with negatives, since in the iatter case the disappear- 
ance of the milky backing is an indication of the speed of fixing; 
whereas, there is no such indication in the case of the fixation of 
prints. For this reason, it is advisable to keep accurate record of 
the number of prints fixed in a given volume of solution, in order 


KQ 


Fig. 194. Electrically Operated Print Washer. (Pako) 


that the latter may be discarded as soon as the limit of its fixing 
powers has been reached. One gallon of any standard fixing bath 
should fix at least a gross 5/7 prints or approximately 5000 square 
inches of paper surface. As soon as this amount is reached, the bath 
should be discarded and a new one substituted. Never add new fix- 
ing bath to a used solution. Pour out the old bath and replace with 
new. The following is a good formula for the fixing bath: 


Y Hypo? ey ee Sa as 16 oz. 250 gm. 
Waater. to makes. 6.58 vee eps wei eae 64 oz. 1000 cc. 


Dissolve and then add the following hardening solution, which may 
be made up in stock solution: 


Sodium sulphite dry... >. ..<a.cdemeteten a \% oz. 3I gm. 
Acetic ‘acid 28 per cént.. 30.5 sind ee ee 3 OZ. 186 cc. 
Powdo-ahumn i) i Uo wee ee ee Y Oz. 3I gm. 
Waterton make ie aa ve-0 2 f oitns ae + ee ee ee 5 oz. 412-06; 


PRINTING PROCESSES WITH SILVER SALTS 405 


Washing.—Prints should be washed for one half to one hour in 
running water and must be kept separated or they will cling together 
and the hypo will not be thoroughly eliminated within this time. Sev- 
eral ingenious machines have been introduced for this purpose. Fig- 
ure 194 shows one of the turbine washers such as are used in large fin- 


= i es 20 Bers 7 
es gz 


Fic. 195. Centrifugal Water Pressure Type of Print Washer. (Halldorsen) 


// 
ij 
== Si 
at } 
Hy} 
VA 
i] 
H}) | 
WIP =" 
rl 
D 


ishing plants. The machine is operated by electricity and has a ca- 
pacity of about 250 to 300 average size prints, which are kept sepa- 
rated and in motion very efficiently. Another type of washer which is 
more suited to the amateur and small worker is that illustrated in 
Fig. 195. The prints are given a swirling motion by the water, which 
enters on the side so that they are kept well separated. The water 
passes out through the siphon arrangement in the center which re- 
moves the hypo-laden water from the bottom and at the same time 
owing to its conical shape prevents the prints from collecting in the 
center. The perfect washing machine does not exist and after all, 
perhaps the most efficient method consists in constantly transferring 
the prints by hand from one large tank to another in both of which 
fresh water is kept running. At any rate, it is well to take no chances 
with imperfect washing and where absolute permanency is desirable, 
tests for the presence of hypo should be made by any of the methods 
previously given in the chapter on Fixing and Washing. The per- 
manganate test may be recommended as convenient and sufficiently 
reliable. 

Drying.—Small batches of prints may he laid out on blotters to dry 
or stretchers covered with cheesecloth or muslin may be used. Prints 
which are allowed to become bone dry in such condition will curl con- 
siderably and many remove prints when nearly dry and place them 
between blotters under slight pressure in order to cause them to dry 
flat. For commercial establishments one of the many forms of drum 


~s — & Dd eee 
Pls be sg! 
’ ; Ww >: 
f e) 
7 Pe 


406 PHOTOGRAPHY 


dryers illustrated in Fig. 196 is recommended. The prints are placed 
between two broad belts which carry them around a heated drum 
about three or four feet in diameter. The revolution requires about 
three to five minutes and when the prints reach the starting point they 
are thoroughly dried. In some models the drum may be run at dif- 
ferent speeds so that single and double weight or thick papers may 
be dried in one revolution of the drum. There are several dryers of 
this type on the market. The one illustrated is the Sickle and is 


; 
a 
3 
-* 
‘ 
4 
| 
q 
4 


me chains sie aay 
Pee ge eT ce ee 


. Fic. 196. Rotary Belt Dryer. (Sickle) 

shown not because it is any better than any other but merely as an 
example. Each of the commercial machines has its own distinctive 
features which should be carefully studied by the intending purchaser 
in order that he may be sure that he is securing the best apparatus 
for his particular requirements. 

Alteration of Contrast.—With a metol-hydrochinon developer it is 
possible to secure varying degrees of contrast with the same paper by 
varying the proportion between the two developing agents. Metol 
being a member of the soft-working class of developers, increasing 
the amount of metol leads to softer results while increasing the pro- 
portion of hydrochinon (which is a contrast developer) leads to 
greater brilliancy. This is at times very convenient when dealing 
with negatives having too much or too little contrast for the particular 
paper required. The three-solution metol-hydrochinon developer 
worked out by Snodgrass is especially suitable for this purpose as it af- 
fords a convenient means of preparing directly from stock solutions 
a soft-working, normal or contrast developer as required. 


seas eS _—— f tc tA Tae 
Oe a ee eee ee Se ee ee a ee ee 


ee ee eae . ane 


Prove tNG PROCESSES WITH SILVER SALTS 407 


With some bromide papers increased contrast may be secured by 
using a hydrochinon-caustic soda developer as employed for process 
and photo-mechanical plates but with some papers and, particularly 
with most gaslight papers, this leads to images of poor color. 

When it is required to secure the best possible results from exces- 
sively contrasting negatives, without resorting to persulphate reduc- 
tion or other manipulation of the negative, Sterry’s method may be 
used.® By its use soft results may be obtained with the very hardest 
negatives. The exposed paper is bathed for two or three minutes be- 
fore development in a solution of potassium bichromate and then de- 
veloped in the ordinary way. The following stock solution is made 


up: 


oe En ES agstel s\n a ara I OZ. 100 gm. 
Nm ene VAIINONIA (2500) so... a else ee ee eee ee 1A: Bg 12.5 Cc. 
yc ne Ga aa IO OZ. 1000. CC. 


For use take one to two drams of the above stock solution to ten 
ounces of water (12.5-25 cc. to 1000 cc.). 

Determine the exposure required to secure the proper detail in the 
highlights (neglecting the shadows) when developing in the usual 
way. Then make up the solution as above and immerse the exposed 
sheet of paper in the solution for three minutes. Wash for half a 
minute and develop in the regular developer. Development is some- 
what slower than ordinarily but the shadows are held back while the 
highlights come out to proper depth sooner so that the print is softer 
and has a better scale of values. An acid fixing bath must be used 
for fixing to avoid stains from the bichromate solution. Various de- 
grees of softness may be secured by altering the strength of the bi- 
chromate solution; the stronger the solution the softer the result, 
other things being equal. 

Reduction and Intensification of Prints.—It is not often that one 
desires to go to the trouble of reducing or intensifying prints as it is 
usually as simple and more satisfactory to make them over. There are 
times, however, as in the case of a big enlargement, where expense is 
an item of importance, when it may be desirable to attempt reduction 
or intensification before going to the time and expense of making a 
new print. 

As in the case of negatives, prints may be reduced so that the con- 
trasts are increased (subtractive reduction), diminished (super-propor- 


5 Phot. J., 1907, 47, 170. 


408 PHOTOGRAPHY 


tional reduction) or unaltered in contrast, the depth of the print alone 
being reduced (proportional reduction). 

For proportional reduction the following permanganate reducer is 
quite satisfactory: 


Potassium: ‘permangatiate 22.2 cwen se eee ea eee 7 ot. I gm. 
Sulphuric acid (10 per cent solution)............ 350 min. 50 cc. 
Woateriiriesh os sis ees Oo led pee ees -4 ROSE 1000 cc. 


This acts rapidly; from 45 to 120 seconds is the average time and it 


reduces any slight fog which may be present. 

For increased contrast (subtractive reduction) the iodine-cyanide re- 
ducer is suitable. Owing to its poisonous nature it must be handled 
with care. 


Iodine (10 per cent solution in potassium iodide 


SOMItION)  ssavd ne oh ee eee ee ee Se 403 min. 57.5 cc. 
Potassium cyanide (10 per cent solution)....... 70 min. 10° ce. 
Weatet! vsadscen ng'Gs aie cae om ae oe 16 oz. 1000 CC. 


For reduction of contrast ammonium persulphate appears to be the 
only suitable reducing agent. 


Ammonium persulphate................++++.-560 gr. 80 —s_ gm. 
Sulphuric atid... .c.csleaoe sees robes poe ee 8 min. 2) ee ee. 
Sodium chloride’ (Salt)... os..« 0s ae eee 6 gr. 0.8 gm. 
Water 23 aS oe ad ee eee 16 oz. 1000), Ce 


For use, dilute with two parts of water.® : 
The most satisfactory agent for intensifying prints is chromium. 
While other intensifiers will produce some increase in depth, the color 
is nearly always affected and the whites of the print tinted while none 
is so dependable in use as chromium. In general the operation is ex- 
actly the same as for negatives. Particular attention, however, must 
be paid to the thorough removal of every trace of bichromate before 
development. The time of washing necessary for this may be con- 
siderably shortened by immersing the print for five minutes in a dilute 
solution of potassium metabisulphite. Redevelopment may be with 
amidol or with the developer used for the original print, provided it is 
fresh and does not contain a large amount of soluble bromide. Should 
the amount of intensification secured the first time be insufficient the 
operation may be repeated. In this case it is well to use amidol for 


6 The above formule are taken from the paper of Jones and Fawkes on the © 


“ Reduction of Developed Prints,” Brit. J. Phot., 1921, 68, 275. 


one 

. Fi gl OG 

ci 5 n “. Sek Fue, ‘ = i ekg : r ioe ~ ae oe 

Per eS ee ATRL ee ee eR I EI RMN Re Ce hy Ge ee eM: ee ee i emp ee ae 
ae ae Re et, Pe eT eee eRe | eee See Meee ee ae Oe Ae i | vo le oN en ae OR Se 


— 


PRINTING PROCESSES WITH SILVER SALTS . 409 


redevelopment as there is less danger of frilling or excessive softening 
of the gelatine. 

The Glazing of Prints.—Prints with a highly glazed surface can be 
produced on any of the glossy grades of P-O-P or developing papers. 
Such prints are best for reproduction and for work of a scientific na- 
ture where it is necessary to preserve fine detail. For producing the 
highly glazed surface, specially prepared iron plates coated with a hard 
brilliant enamel are generally employed. These are commonly termed 
ferrotype plates or squeegee tins. The wet prints are placed on the 
polished surface of the ferrotype plate and brought into perfect con- 
tact with the same by the use of a wringer or a hand roller. When 
dry, they can be stripped from the plate, having acquired as a result 
of this treatment a highly glazed surface similar to that of the plates 
on which they were dried. In place of the usual ferrotype plates, 
celluloid or glass may be used. 

The highest glaze is secured with glass, but owing to the greater 
danger of prints sticking to the surface so that they cannot be removed 
without damage, either the ferrotype tins or celluloid are preferred. 

The first aim of the worker should be to learn the characteristics of 
the paper used with respect to glazing. Some papers, which are har- 
dened in the process of manufacture, may be removed directly from the 
wash water and placed upon the tins; with other papers this would re- 
sult in complete failure and some form of prehardening is necessary to 
prevent the prints from sticking to the plates. If the prints are dried 
and then resoaked until limp in cold water, the gelatine becomes con- 
siderably harder and the danger of prints sticking to the plates is 
largely obviated. Either alum or formaline may be used for harden- 
ing the emulsion, in order to prevent the necessity for an intermediate 
drying, but of the two formaline is undoubtedly the better. In the 
first place it does not endanger the permanency of the prints, as if not 
completely washed out in the few minutes wash which should follow 
immersion in the formaline solution it will evaporate entirely.. In ad- 
dition, it possesses the advantage over alum in that it has no tendency 
whatsoever to produce iridescent markings on the prints. A solution 
of 1 oz. formaline to 20 oz. water is sufficiently strong. 

If glass is used the plates are first cleaned by soaking for several 
minutes in weak sulphuric acid (1 oz. commercial H,SO, to Io oz. 
water), then rinsed under the tap and scrubbed with plain washing 
soda, again rinsed and allowed to dry. When dry the glass is coated 
with perfectly fresh ox-gall (1 oz. ox-gall to 40 oz. water). Old ox- 


410 . PHOTOGRAPHY 


gall is worse than useless and will actually cause the prints to stick. 
The prints are placed face down on the prepared surface, a blotter 
placed over them and pressure applied by means of a roller until the 
prints are seen to be in perfect contact with the surface. The glass is 
then placed in a cool dry place and the prints allowed to become thor- 
oughly dry before trying to remove them. Some may leave the glass 
completely when dry, those that do not may be removed by inserting 
a finger nail under one corner and pulling away from the glass. 
Lately Callier has recommended that the glass be coated first with 
a 2 per cent solution of gelatine. When this is dry, a thin film of 
collodion is superimposed. ‘This collodion is prepared as follows: 


Pyroxyline (soluble) csc.4.5 sak jo as pee 1% oz. 45 gm. 
Vasoline (oil os. ercssa eis ooee wheat ee ee 35 win: 2 £8; 
Amyl acetate’ to make. ..5...%.. issue ee eee 35> OZ, 1000 cc. 


When the collodion has dried the prints are applied as usual and when 
dry will drop off. There is absolutely no danger of sticking. ; 

Ferrotype tins, however, are more generally employed in this coun- 
try. With these the danger of prints sticking to the plates is much less 
than with glass. It is necessary, however, to keep them absolutely 
clean and well polished. For this purpose a solution of benzol and 
spermaceti wax, or turpentine and beeswax, .is usually employed. 
Suitable formulas follow: 


Beeswax’ “20.5 Geel ol ee Coe ee 20 gr. 45 gm. 
Turpentine 455 fi 5 uce ee ee aoe eee I Oz. 1000 cc. 
Sperniaceti “waxviy 1219) 4 ink, a 20 gr. 45 gm. 
Benzole =. a. .3 cia veh ae ae i Oz; 1000 cc. 


A few drops of this are rubbed on the plate with a piece of clean flannel 
and the tin polished with another piece of flannel or other soft cloth. 

Celluloid forms a very suitable substance for glazing although the 
gloss is not so high as that produced by glass or by ferrotype plates. 
A special brand known as Glazine, manufactured by the Glazine Pad 
Company, Hillsborough, Sheffield, England, is supplied for this pur- 
pose. With this the danger of prints sticking to the plate is practically 
negligible. Neither is polishing of any kind required. ‘The writer has 
used them with perfect success for two years and believes them to be, 
all things considered, the most satisfactory glazing substance obtainable. 


PRINTING PROCESSES WDESSIEVER SALTS ).411 


PRINTING-OQuT PROCESSES 


Gelatine Printing-Out Papers.—Gelatine printing-out papers 
(called P-O-P for short), once almost universally used for positive 
printing, have been rendered almost obsolete by the modern developing 
papers and are now seldom employed. There remain, however, some 
purposes for which they are still unrivaled as, for example, for prints 
from which half-tone plates are to be made and for photo-mechanical 
reproduction processes generally. It seems well, therefore, to include 
some brief directions for the manipulation of such papers. 

Printing itself is very simple and calls for but little comment. The 
printing frame may be loaded and the progress of printing observed in 
an ordinary room if one takes the precaution not to stand close to a 
window and does not leave the paper exposed to the direct rays of light 
any longer than absolutely necessary. For the best result the negative 
must be one which hassreceived full exposure and development; weak, 
under exposed or under developed negatives will not make good prints 
on gelatine printing-out paper and such negatives are best printed on 
developing paper. Care should also be taken that the negatives are 
perfectly dry, otherwise a silver stain may be formed which it is al- 
most impossible to remove. 

Exposure should be to the strongest daylight available, excepting 
direct sunlight, which must not be used except with very dense and 
contrasty negatives. As there is a certain decrease in depth in the 
processes of toning and fixing, printing must be carried considerably 
further than would appear necessary from an examination of the pic- 
ture in the printing frame. The depth to which printing must be 
carried to allow for this falling off in toning and fixing is easily 
learned after a few trials and thereafter gives no trouble. After being 
removed from the frame the prints are placed away from light and 
under pressure until a number have accumulated and one is ready for 
toning and fixing. Exposed prints, however, should not be kept from 
one day to another. 

Toning.—Before toning, the prints are washed in running water 
for a quarter of an hour, or in several changes of water, care being 
taken that washing is thorough and that each print receives its proper 
share of washing. Prints which are left lying on one another do not 
wash properly no matter how long the period of washing; it 1s essen- 
tial that they be kept moving, so that each print is exposed to fresh 
water. Some methods of toning, however, do not require the previous 
washing. 


412 PHOTOGRAPHY 


There are almost innumerable formulas for the toning bath and 
many variations in the processes of toning. As ordinarily practiced, 
toning is an operation which requires considerable practice in order to 
be able to secure satisfactory and uniform tones. There is a method 
of controlled toning, however, with which even the most inexperienced 
person can secure agreeable tones and with a high degree of uniform- 
ity. The solutions required are a 10 per cent solution of ammonium 
sulphocyanide, a 10 per cent solution of common salt, a 10 per cent 
solution of hypo and a gold bath containing 1 grain of gold chloride to 
each dram of water. The principle is to use a definite weight of gold 
for a given number of square inches of paper and leave the prints in 
the bath until the gold has been used up. 

The toning bath is made up as follows: Measure out I0 ounces 
(1000 cc.) of water and add two drams (25 cc.) of the sulphocyanide 
solution and 1 ounce (100 cc.) of the salt solution. Mix, and add 1 
dram (12.5 cc.) of the solution of gold chloride. Label the bottle 
Gold Toning Bath. Each ounce (28.4 cc.) of this solution contains 
I-10 grains (.0064 gm.) of gold which is sufficient for two 34 x 4%4 
prints. For warm brown tones half to three quarters of an ounce 
(14.2-21.3 cc.) is enough: for blue tones a little more may be needed. 
Other sizes may be handled by taking the proper amount for the size 
of the print. } 

Now suppose you have ten 3% x 4% prints to tone. Measure out 
5 ounces (142 cc.) of toning bath and put the prints directly into it 
without washing. Continue to move them around in the bath until no 
further change of color can be observed. The final stage is when the 


surface looks cold and slaty blue. They are then removed, washed, | 


fixed and again washed and dried. 

Instantaneous Toning.—Another certain method of securing uni- 
form tones is the so-called instantaneous method. Four stock solu- 
tions are required: 


A, Ammonium sulphocyanidé.<..-.2. oes 1 ae 100 gm. 
Water ‘to make: 0s 0¢s e¢ ace oe ree ee iO Oe 1000 cc. 
8, Sodium phosphate, ac, - sees ate oes ee Lae 133.3 gm. 
Water to make. .2 2. cae eee ee 74 oz. 1000 CC. 
C. Sodium phosphate...... RON mene Lo 3 oes 100) «gm. 
Water -to make: . icco7tae eae eo eee 10. Oz. 1000 cc. 


D. Saturated solution of borax. 


PrN TING PROCESSES WITH SILVER SALTS 413 


Mix for ten 4.x 5 prints (200 sq. inches), 


Renner es 52S. A PR Pest ds, 10 parts 
Wiyateeche ceerk ik Y > GRE RDI ELs SRST a GA Le PHS Oise cou teh Lie) Oz; 80 parts 
Td ee EDs alte pa 5 fit dieses Y% Oz. 5 parts 
NN 8 eas 5 pe ety Se chal Sn hhw on aim Puicdr; 10 parts 
PM et Ny Sak is og eke civ'dies ecnla a's « 2s Ot; 20 parts 


The prints, which should be only one shade darker than the desired 
shade, are put directly in the toning bath without previous washing. 
On entering the bath they first turn red, then a dark purple which is 
almost black in the deepest shadows. No matter how much longer 
they are left in the bath no further change takes place. As soon as 
toning is seen to be complete, the prints are fixed or removed to a tray 
of clear water until ready for fixing. 

The advantages of these methods over those commonly advised are 
obvious as all uncertainty is removed and the operation can be worked 
at night by artificial light which is impossible in the ordinary way. 
The editor of American Photography remarks that a large number 
who have given it a trial found it to work perfectly.” 

Black Tones with P-O-P.—Black tones can only be secured with 
P-O-P by toning with platinum. The following bath should be used: 


MUPPIPOMONVIENEAIAMINE ©... 06.0... 6.0s bee eee e ss Tea, 1.4 gm. 
Preeti Cioropiatinite........ 00... eda eae es 7 or. 1.4 gm. 
IA he lk ccs bya de ew acne seace 10 62. 1000 CC. 


This solution must be prepared directly before use as it does not keep 
at all well. As soon as the desired tones are secured remove the prints 
and then fix and wash thoroughly. Bluish-black tones may be secured 
by first toning in a gold bath, washing well and retoning with platinum ; 
the color depending on the depth to which gold toning is carried. The 
more gold deposited the bluer will be the final result after platinum 
toning. 

When a platinum toner is used, the prints should be immersed before 
toning in a 5 per cent solution of salt for five minutes, then rinsed and 
toned. After toning the prints should be immersed for 5 minutes in 


(ce Gi he RS ue Ngd ott 1% oz. 140.4 gm. 
pericnrieatpouace COry) 8 eek aye eee bas 34 OZ. 46.8 gm. 
ee ee is) IDE Fae eu cages 163 Oz: 1000 CC. 


Then rinse, fix and wash thoroughly before drying. 


7 The writer is indebted to Fraprie’s work Practical Printing Processes for 
these methods of toning. 


28 


414 PHOTOGRAPHY 


Fixing.—After toning the prints are washed for several minutes, 
then transferred one by one to the fixing bath which consists of a Io 
per cent solution of hypo. The prints must remain in the fixing bath 
at least 10 minutes during which time they must be separated con- 
stantly by hand in order that the fixation of each print may be thor- 
ough. Fixing is followed by a thorough washing for % hour in run- 
ning water, or in 6 changes of water, allowing 5 minutes for each 
change, after which the prints are ready for drying. 


GENERAL REFERENCE WorKS 


BurBANK—Photographic Printing Methods, 1896. 

GLover—Print Perfection—How to Attain It, 1924. 

HANNEKE—Das Arbeiten mit Gaslicht und Bromsilberpapieren, 1918. 

Hinton—P-O-P. 

Mercator—Der Entwicklungs-druck auf Bromsilber, Chlorbrom und Chlor- 
silber Gelatineemulsion Papieren, 1907. 

STENGER—Neuzeitliche photographische Kopierverfahren. 

SreNcER—Die Kopierverfahren, 1926. 

Snopcrass—The Science and Practice of Photographic Printing, 1923. 

VALENTA—Die Behandlung der fur den Auscopir-Process Bestimmen Emul- 
sionspapiere, 1806. : 


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CA hiro VII 


PROJECTION PRINTING 


Introduction.—There has been an increasing tendency on the part 
of both amateurs and professionals in recent years to abandon the 
large, bulky and cumbersome camera and its necessary impedimenta 
and rely entirely upon projected prints from the small negatives 
where large prints are required. This course has much to recom- 
mend it. Photographic materials are now so perfect that given 
proper care small negatives can be made which will stand consider- 
able enlargement and still compare quite favorably in quality with 
contact prints from larger negatives, while the convenience of the 
small, compact hand camera is not to be belittled. 

Perhaps the principle of projecting will be clearer from an examina- 
tion of Fig. 197, in which A represents the illuminant (either daylight 


Fic. 197. Principle of Projection Printing 


or artificial light), B the negative, C the lens, and D the easel. The 
rays of light from A pass through the negative B, an image of which 
is formed on the easel D by the lens C. The distance between the 
lens and the easel determines the degree of enlargement, the greater 
this distance the greater the degree of enlargement. 

Fixed-Focus Enlarging Cameras.—The simplest form of apparatus 
for projection printing consists of a box carrying at one extremity the 
negative and at the other the sensitive paper, with a lens between 
(Fig. 198). Such cameras are an article of commerce and are 1n- 


415 


416 PHOTOGRAPHY 


tended. primarily for the amateur or occasional worker. As they are 
fitted with cheap single lenses with a small aperture, the time of ex- 
posure is relatively long and may easily run into minutes unless day- 
light and a fast bromide paper are employed. The fixed-focus 


Fic. 198. Box Enlarging Camera 


camera forms a very convenient and satisfactory instrument for the 
man who desires to make a larger print now and then and who does 
not care to go to the trouble and expense of employing a more elabo- 
rate form of apparatus. 

While such instruments may be obtained commercially, there is no 
reason why the worker cannot make one of his own if he is at all 
handy with tools, as the construction is quite simple. For this pur- 
pose one may make use of the lens attached to his regular camera, 
providing the fixed-focus enlarging box with a lens flange to take the 
same, or, if the lens cannot be removed from the camera, fastening 
the entire camera on a platform within the enlarging camera. As the 
commercial forms of such apparatus are equipped only with single 
lenses having a very small aperture it will be readily seen that with — 
but a little time and materials the worker may provide himself with 
apparatus which is actually superior to the commercial article. The ~ 


PROJECTION PRINTING 417 


total length of the camera is, obviously, the sum of the distances 
separating the lens and negative and the lens and sensitive paper. If 
the positions of the nodes of the lens are known the two conjugate 
distances can be precisely measured and the partition carrying the 
lens placed in position without preliminary testing. When the posi- 
tion of the nodes is unknown it is necessary to find the proper position 
for the lens by experiment. In fact to ensure critical focus it is well 
in all cases before fixing the lens to make some preliminary tests on 
a sheet of ground glass placed in the position to be occupied by the 
sensitive paper. 

Apparatus for Projection Printing with Davlight —There are cam- 
eras made especially for enlarging and reducing. These are provided 


Fic. 199. Daylight Enlarging Camera 


with long bellows extension so that the degree of enlargement, or re- 
duction, may be varied, within certain limits, as required. In Fig. 199 
we illustrate a typical camera designed for this purpose. Such cam- 
eras are heavy and expensive, occupy considerable space and, more- 
over, the size of enlargement is limited by the size of the camera. 
For these reasons other forms of apparatus are to be preferred. 
The most satisfactory method of employing daylight for projection 
printing is illustrated in Fig. 200. A window which receives clear 
unobstructed light from the sky is blocked up, except for a small 
opening about twice as large as the largest negative to be used, and 
provision made for attaching an ordinary camera, the reversing back 
having been previously removed. A platform is built to support the 
camera and guides or markers placed to show the proper position of 
the easel. Provision must of course be made for holding the nega- 
tive and one or two sheets of ground or opal glass which serve to 
diffuse the light and prevent uneven illumination of the negative. 
Where clear unobstructed light cannot be secured it is necessary to 


418 PHOTOGRAPHY 


employ a reflector, as shown in the figure. This reflector must be at 
an angle of 45° so that it will cast the unobstructed light from the 
sky onto the negative. This reflector may be of any enameled surface 
as white paper, or wood painted with a glossy white paint. A mir- 
ror is to be avoided. In place of a reflector the ribbed glass known 


es 


im 


Fic. 200. Projection Printing Apparatus for Use with Daylight 


as prism glass may be used, being placed two or three inches before 
the negative in order that there may be no danger of it being in focus. 
Daylight has several positive advantages and at the same time disad- 
vantages which prevent its general use. In its favor it may be said 
that the light is rapid and owing to the perfect diffusion any retouch- 
ing or handiwork on the negative does not show so prominently as 
when the light comes from a concentrated source. For this same 
reason there is less granularity apparent with a high degree of enlarge- 
ment when daylight is used than when condensers and a concentrated 
light source, such as the electric arc, are employed. On the other 
hand, daylight is never constant and exposures are therefore liable 
to sudden change, which occasions considerable waste of time and 
material. 

Apparatus for Projection Printing Using Artificial Light. 
to the inconstancy of daylight practically all enlarging is now done 
with artificial light. The typical lantern for enlarging with artificial 
light is illustrated in Fig. 201. It will be observed that this consists 
of the illuminant with its light-tight lamp house, either condensers or 
reflectors for securing the maximum efficiency of illumination from 


PROJECTION PRINTING 419 


the source used, the negative carrier, the bellows with a projecting 
lens and lastly the easel. While this is the schematic plan of prac- 
tically all enlarging lanterns on the market, they naturally vary a good 
deal in minor details. The construction of such apparatus is not 
above the capabilities of the average worker who is handy with tools 


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Fic. 201. Enlarging Lantern for Artificial Light 


and should he care to use the lens and camera which he has already, 
the expense of the apparatus may be made almost negligible. Direc- 
tions for making such equipment have been published many times in 
nearly all of the journals to which the worker who wishes to build his 
own lantern is referred for further information. 

Where condensing lenses are not employed and the light source is 
one which does not require attention while in use, it may be convenient 
under certain circumstances to place the light outside of the room used 
for enlarging. This arrangement has the advantage of increasing 
the amount of floor space and lessening the effect of the heat liberated 
by the lights. 

Self-Focusing Apparatus for Projection Printing.—A new era in 
apparatus for projection printing began with the introduction by the 
Eastman Kodak Company in 1920 of self-focusing projection ap- 
paratus in which the image is kept in focus automatically regardless 
of the degree of enlargement. Other manufacturers have followed 
the Eastman Company in the field and a number of models are now 
available. Among these may be mentioned the Callier; Ica; Border- 
tint; Aldis-Ensign ; Sichel’s Overton; Butcher’s Autoprint; Noxa, be- 
sides many others. Directions for the construction of self-focusing 
projection apparatus have been published in several places (see bib- 
liography ). 

Illuminants for Projection Printing.—The principal requirements 


420 PHOTOGRAPHY 


of a satisfactory illuminant for enlarging are that it should be reason- 
ably constant in strength, that it should be strong enough to allow of 
rapid exposures, and be easily adjusted and convenient in use. Day- 
light is the least suitable of the illuminants because of its variability. 
It varies not only from day to day and from hour to hour but may 
even vary considerably in the short space of a few minutes. For this 
reason daylight is entirely unsuited to the professional or commercial 
enlarger who must produce prints of uniform quality, while its use 
by the amateur means the wastage of much material that might other- 
wise be saved. The worker who has access to electricity will of 
course use some form of electric light, and, while it must be admitted 
that equally good enlargements may be made by daylight or the 
weaker light sources as acetylene or gasoline vapor lamps, electric 
light sources surpass all others in general adaptability, since on the 
whole they are more constant in intensity, more easily adjusted and 
possess higher intensities than the other sources. 

The Mercury-Vapor Lamp.—One of the most satisfactoey lights for 
enlarging is the M-shaped mercury vapor tube supplied by the Cooper- 
Hewitt Electric Company of Hoboken, N. J. The light is extremely 
rich in violet rays and is consequently very rapid, so that the slower 
grades of “ gaslight ” paper may be used successfully, while for large 
projected prints the time of exposure is materially lessened. The M- 
shaped tube gives an even illumination which requires no condensers 
and, as little heat is produced, it is particularly suitable for summer 
use. Owing to the M-shape of the tube the illumination is so much 
diffused that the minimum amount of ground or opal glass is required 
to secure even uniform lighting. This results in a higher intensity. 
Undoubtedly the M-tube with a sheet of ground or opal glass as a 
diffuser forms the nearest approach to daylight of any artificial light 


and is especially desirable for enlarging from portrait negatives. The 


mercury-vapor lamp is a rather expensive illuminant for the amateur 
as its initial cost is rather high, but as it consumes very little current 
it is not so expensive in the end and in the long run is well worth the 
cost. Where much enlarging is done and speed and quality and not 
the initial expense are the prime requisites, then the mercury-vapor 
lamp may be considered the ideal light. 

The Electric Arc.—From the optical standpoint no other light so 
completely fulfills the requirement of the ideal light surface for en- 
larging as the electric arc. Whereas all other sources radiate light 


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PROJECTION PRINTING 421 


in all directions, the light from the arc is confined to a-small area of 
about half an inch in diameter on the positive carbon. The light is 
extremely intense and also very rich in blue and violet rays, especially 


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Fic. 202. Proper Position of the Carbons of an Arc Light for Use with 
Alternating Current 


if flaming carbons are employed. Direct current is by far the most 
satisfactory for an arc light source as it is more steady, under better 
control and more economical of current. If alternating current must 
be used, then it is well to arrange the light in such.a way that the 
carbons are fitted toward the condensers at an angle of about 30° (Fig. 
202). When this is done the highly incandescent tip at the extremity 
of the lower positive carbon is directed toward the condensers, result- 
ing in a higher intensity of illumination. Were it not for its incon- 
stancy and the attention which it requires, the arc lamp might be con- 
sidered the ideal source for projection printing, but these are factors 
of considerable importance in practice and consequently we believe 
that for general purpose work either the mercury vapor tube or gas- 
filled mazda lamps are to be preferred. 

Incandescent Lamps.—Gas-filled lamps of high intensity, such as 
500, 750 and 1000 watts, have been extensively used for projection 
printing since their introduction several years ago and are steadily in- 
creasing’ in popularity. They are very constant and convenient in 
use, and the color of the light, while not as rich in actinic rays as the 
arc or mercury-vapor lamp, is nevertheless very satisfactory and the 
intensity is sufficient for all ordinary enlarging. Special types, known 
as stereopticon or projection lamps, are now supplied having a small 
concentrated filament which is almost as satisfactory for use with 
condensers as the arc. On the whole, however, lamps of this type 


422 PHOTOGRAPHY 


work better with parallax reflectors than with condensers. The prin- 
cipal objection to the gas-filled lamp is the amount of heat produced 
and, as in the case of the arc, which develops still more heat, care 
should be taken that perfect ventilation is secured, otherwise the nega- 
tive may be melted or warped especially if on film. 

Although of lower intensity than electric sources, the Welsbach 
mantle is well suited to projection printing except for high degrees 
of enlargement, or with dense negatives, when the time of exposure 
may be rather lengthy. For those who do not have access to elec- 
tricity or gas, acetylene forms the only satisfactory form of artificial 
illumination for which apparatus is available in this country. We il- 
lustrate in Fig. 203 a special acetylene burner supplied by Burke 
and James. It will be observed that the four flames are brought to- 


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Fic. 203. Acetylene Burner for Projection Printing. (Burke and James) 


gether in close formation so that a concentrated light is obtained which 
equals in intensity that obtained with six or eight ordinary burners. 
This burner together with a reflector and condensers forms a com- 
paratively rapid light and for the man who has access to neither elec- 
tricity nor gas is certainly to be preferred to daylight. 

There are on the British and other markets a number of lamps 
burning methylated spirit which produce a very intense light well 
adapted for projection printing but their equivalent does not appear 
to have been introduced in this country. We illustrate in Fig. 204 two 
of these. The first is known as the Luna light and is supplied by the 
firm of W. C. Hughes and Co., Brewster House, 82, Mortimer Road, 
Kingsland, London N. i and the second, the Radio spirit lamp, is sup- 
plied by Brinkman, 26-27 Hatton Garden, London E. C. 1. 


PROJECTION PRINTING 423 


Writing in the British Journal of Photography (1922, 69, 767) W. 
Gard described how he has used for a 34x 4% enlarger a 4-volt, 4- 
watt lamp in connection with a 6-volt, 40-ampere-hour accumulator 
and for a 44% x6¥% enlarger a 6-volt, 12-watt lamp with an 8-volt, 
40-ampere-hour accumulator. The lamp is of lower voltage than 


Fic. 204. Lamps Burning Methylated Spirit for Projection Purposes 


the accumulator and thus gives a more powerful light. He states 
that an exposure of 4 seconds was found quite sufficient when en- 
larging from 44% x6% to 8x10 or 10x 12 using ordinary bromide 
paper. 

Securing Even Illumination without Condensers.—Whatever the 
illuminant employed it is of primary importance that the negative be 
evenly illuminated. For this purpose condensers are generally em- 
ployed with the electric arc and other illuminants approximating to a 
point source, such as the Nernst lamp or the low volt, concentrated 
filament, locomotive headlight lamp. For more diffused sources, re- 
flectors are more suitable. 

For gas-filled mazda lamps of 500 and 1000 watts the Parallax re- 
flector provides a convenient and efficient means of securing uniform 
illumination without diminishing the intensity of the source. Fig. 
205 shows the construction of this reflector and its appearance when | 
illuminated. It consists of a number of silvered mirrors so placed 
that the rays of light from the lamp which strike them are reflected 
out towards the negative in a parallei beam. 

A very satisfactory means of dealing with incandescent lamps of 
lower power is to employ the same in series as shown in Fig. 206. 


424 PHOTOGRAPHY 


The lights at the corners should be somewhat stronger than that at 
the center which merely serves to fill in the gap in illumination in the 
center. This arrangement used with one or two sheets of ground 
glass should provide perfectly uniform illumination. 

Although less powerful than direct light there are several worth- 
while advantages to the use of reflected light. It is softer, owing to 


Fic. 205. Parallax Reflector for Use with Incandescent Electric Sources 


more complete diffusion, and consequently gives softer, more delicate 
results, with a longer scale of gradation and greater freedom from 
granularity than a direct source. For the portrait photographer it is 


Fic. 206. Securing Even Illumination with Five Incandescent Electric Sources 


particularly advantageous since any retouching or other handwork 
on the negative does not show up so prominently as when a direct, 
concentrated source is employed. | 

While theoretically the reflector to be employed should be parabolic 
in form, in practice any of the forms illustrated in Fig. 207 will be 
found suitable. The light houses illustrated may be built of thin sheet 


PROJECTION PRINTING 425 


metal or wood. In the latter case it is well to line the sides adjacent 
to the lights with asbestos to prevent danger of fire. The reflecting 
portion should be of sheet iron coated with aluminum paint. Care 
should be taken that the reflector is not too small or the illumination | 
of the negative will not be uniform. To ensure uniform illumination 
the reflector should be made at least 3 times the size of the largest 
negative to be employed. For the smaller sizes a single gas-filled 
bulb on each side may be sufficient. For larger sizes, however, it 


Fic. 207. Forms for the Lighthouse Using Reflected Light. (Wall) 


will be necessary to use more than one bulb on each side, or the long 
tubular lamps as used in window display illumination may be used. 

The Condenser in Projection.—With a suitable light source the 
negative can be uniformly illuminated by means of condensing lenses. 
These lenses are of the plano-convex type and are used in pairs, the 
two convex sides facing one another and separated by a fraction of an 
inch. The diameter of the condensing lenses must be at least as large 
and preferably somewhat greater than the diagonal of the largest nega- 
tive with which they are to be employed. Condensing lenses are sup- 
plied in pairs either mounted or unmounted in the following sizes and 
focal lengths : 


426 PHOTOGRAPHY 
Diameter Focal length in inches Thickness in inches 
AO eg ee eT ee ee cy ee Be eR 54 1R 
ier eras Sli has Sets Ce ee 51 1340 
4% i) Spoink Nh deat eat at er ey Aunte eRe ea yaar p nes 61% 2% 
Ire eA Nay Oey 1) ARC ony Rehr DP ER ee ChE OW ae Cth: = 6% 1549 
ae ee cS Bo oS ot ia Vem 8 1540 
RONG ein, eck sia Soe Fite ea eee 10 rA6 
ER Le a clad y P ales Ca aoa eee 10 14 
Tee PG Fiche came OX a A eae 12 11545 
DR aes eee S834 See 14 158 
BG ee Oe CLE EE 15 1l%%o 
Pian Cet eae ls oe ia rae 18 2% 
De hee ee eh ong Sate acne ry lets 21 25% 


The focal length of a pair of mounted condensing lenses can be de- | e 
termined from the usual rule governing the focal length of a com- 
pound lens system. As the plano-convex lens has only one determin- 
able nodal plane, which lies adjacent to the vertex of the convex: sur- 
face, the distance between the nodal planes obviously becomes that 
separating the convex surfaces of the lenses. The expression thus | 
reads: eee 
fy X Fe 


Po hae 


Where: F =the equivalent focal length. 
F, = the focal length of lens 1. 
F, = the focal length of lens 2. 
d ==the distance between the facing convex sides of the 
mounted lenses. 


X 


The function of the condensing lenses is to collect the diverging 


Fic. 208. The Function of the Condenser 


rays of light and condense these on the negative, bringing the light 
rays to a focus in the projecting lens. Thus in Fig. 208 the light rays 
from the source L strike the first condenser and are refracted so that 4 


PROJECTION PRINTING 427 


they enter the second condensing lens parallel to one another. In this 
second condensing lens further refraction takes place, the rays being 
converted into a converging cone the apex of which (or the focus) 
lies at, or very near to, the principal nodal plane of the projecting 
lens, O. Thus the rays which in the absence of the condensing lenses 
would have continued on in straight lines and be lost are bent and 
made to pass through the lens, so that uniform illumination together 
with a high intensity is obtained. 

From this it is evident that to secure the maximum intensity, to- 
gether with uniform illumination of the negative, when condensing 
lenses are used without a diffusing medium, such as ground or opal 
glass, the light source must be located at such a point on thé optical 
axis that its image, formed by the condensing lenses, is located at the 


way G--~ +s = +2 
Fic. 209. Conjugate Foci in Enlarging 


mpss WO 


principal nodal plane of the projecting lens. If we ignore the exact 
nodal planes, this means that the distances 4B and CD (Fig. 209) are 
conjugate foci of the condenser while the distances ED and DF are 
conjugate foci of the projecting lens. As the degree of enlargement 
is altered the distances ED and DF will vary according to the rule of 
conjugate foci (Chapter III, pp. 68-71). With every variation in 
ED : DF there will be a corresponding variation in 4B : CD, so that 
in order to keep the image of the light source in its proper place at 
the principal nodal plane of the projecting lens the distance AB must 
be varied each time the degree of enlargement is altered. 

The problem is thus one of conjugate foci and as such might be 
calculated mathematically with the aid of the formulas given in an 
earlier chapter (Chapter III, pp. 68-71). However, owing to the 
fact that condensing lenses are of cheap construction and entirely 
without correction, the image of the light source is never a sharp one 
and in practice such calculations are not of much practical value. 
One can usually determine the proper position for the light source 
with sufficient accuracy for all practical purposes from the examina- 


428 PHOTOGRAPHY 


tion of the circle of illumination as thrown on a sheet of white paper. 
The character and position of colored fringes or areas of uneven il- 
lumination indicate the steps which should be taken towards securing 
equality of illumination. This matter is illustrated in Fig. 210. 


Fic. 210. Adjustment of the Light Source with Condensers 


In Figs. 1 and 2 the radiant, i.e, the crater, needs to be properly adjusted 
laterally; it is too far to the right or left. 

In Figs. 3 and 4 it is too high or too low. 

In Figs. 5, 6 and 7 it is too near or too far from the condenser. 

Fig. 8 shows it to be in correct position, the field being entirely clear. 


Condensing Lenses with Diffusing Media.—With a light source of 
small dimensions, such as the electric arc, and in the absence of a dif- 
fusing medium, such as ground or opal glass, the image formed at 
the focal point may be smaller than the aperture of the objective. It 
is obvious that in such cases the effective aperture (on which the 
speed of the lens depends) is not that indicated on the mount but that 
of the light beam which passes through the objective. Thus it may 
happen that a lens with relative aperture of F/4.5 is actually being 
used at a relative aperture of //11. Therefore the time of exposure 
would not be altered if the lens is stopped down to F/6.3 or F/8. 
However, if a diffusing medium, such as ground or opal glass, be 
interposed in the path of the light rays, either in front of or between 
the condensers, this condition no longer applies. One is then dealing 
not with direct light but with scattered light and in this case the ex- 
posure is directly proportional to the lens aperture as in ordinary 
photography. The ground or opal glass may be placed either in front 
of or between the condensing lenses. Theoretically placing the dif- 
fusing medium between the condensing lenses results in less loss of 
light from diffusion, but practically the difference is so small as to be 


pare 


PROJECTION PRINTING 429 


almost negligible, while there are other disadvantages which far out- 
weigh this slight advantage. The uniformity of illumination is not 
nearly as satisfactory as when the diffusing medium is placed before 
the condensing lenses and in the case of ground glass the grain tends 
to produce a granular effect which may in certain cases be decidedly 
cbjectional and which it is always well to avoid as much as possible. 
This is due to the fact that the ground glass is much nearer to the 
focal plane of the negative and therefore more nearly in focus. 
Therefore with ground glass a position slightly in front of the con- 
densing lenses is advisable. Opal glass being practically free from 
granularity may be placed between the condensing lenses if desired. 
Even in this case, however, a position before the condensers is prob- 
ably to be preferred. The use of ground glass makes unnecessary the 
adjustment of the light source each time the degree of enlargement 
is altered. However, in order to obtain maximum printing speed, an 
adjustment should be made when there is a considerable difference in 
the degree of enlargement. By the use of diffusing media, scattered 
rather than direct light is employed and this, as already noticed, has 
the effect of reducing to some extent the contrast of the large print 
as well as its granular appearance. For these reasons the use of dif- 
fusing media is always advisable with portrait or pictorial negatives 
and for those to be enlarged considerably. 

The Projection Objective—A large number of the projection ap- 
paratuses on the market, especially those of foreign make, are fitted 
with objectives of the Petzval type. Aside from its large aperture 
and excellent axial spherical connection, this type of lens is by no 
means the best for the purpose, owing to the rapid diminution in the 
intensity of the image towards the margin and the pronounced cur- 
vature of field. Much better is the aplanat, or rapid rectilinear, which, 
though slower, has a flatter field with more perfect marginal definition. 
The rectilinear, however, is surpassed by the anastigmat whose su- 
perior marginal definition combined with an astigmatically flat field 
renders it particularly well suited for projection purposes. For use 
with condensers in conjunction with an arc, or similar source, and 
without a diffusing screen there is no advantage in the use of a lens 
having an aperture much larger than F/6.8 since this is sufficiently 
large in most cases to admit the whole of the converging beam of 
light from the condensing lenses. However, when condensing lenses 
are used with the ordinary incandescent light, or diffusing media of 

29 


eR ema 
ay an 
4 aes 


430 PHOTOGRAPHY. 


any kind is inserted in the path of the light rays, a larger aperture 
may be required in order to secure the full efficiency of the light 
source, since in this‘case the iniage of the light source formed within 
the projecting lens is larger than before. With completely diffused 
light sources, such as the Cooper-Hewitt mercury-vapor light, groups 
of incandescent lamps used with ground or opal glass, totally reflected 
light or a single incandescent electric light in a parallax reflector, a 


Fic. 211. Loss of Light between Condensers Due to the Use of a Long Focus 
Lens for Projection. (Candy) 


large aperture is also of advantage in reducing the time of exposure. 

Except when condensing lenses are employed the only effect of the 
focal length of the lens is to determine the length of the apparatus 
necessary. The longer the focal length of the lens the greater will 
be the bellows length and floor space for a given degree of enlarge- 
ment. 

With condensing lenses, however, the focal length of the lens must 
be chosen with reference to the focal length of the pair of mounted 
condensing lenses. If the focal length of the objective is much 
longer than that of the condensing lenses, the light efficiency is reduced 
by loss of light between the condensing lenses as shown in Fig. 211. 
In addition to this loss, under such conditions the size of the light 
image will be greater than the size of the light, and the aperture of the 
objective may not be sufficiently large to accept it, which results in 
further loss. Contrariwise if the focal length of the objective is much 
less than that of the condensing lenses, the converging power of the 
latter will be reduced owing to the convergence of the rays between 
the condensing lenses (Fig. 212). Therefore when condensers are 
employed the objective used for projection should have a focal length 
approximately equal to the focal length of the pair of mounted con- — 
densing lenses. Exact equality is not required but the two focal 
lengths should be as nearly identical as possible. 


> a ie [ c 
Ge eee eee ee 


PROJECTION PRINTING 431 


Attention may perhaps be usefully called to the fact that with cer- 
tain lenses in which the front component has a strong condensing 


‘6 


action, so that the “inconstancy of aperture” (Chapter II, page 79) 


Fic. 212. Loss of Covering Power Owing to the Use of Short Focus Projecting 
Lens with Condensers. (Candy) 


is pronounced, better results are obtained when the lens is turned 
so that the front component faces the negative. 

The Projection Easel.—The easel may be simply a large drawing 
board of soft wood to which the paper is attached by means of push 
pins. As a matter of convenience the wood may be covered with 
heavy “cork lino,” a heavy linoleum used for floor covering. This 
enables the paper to be pinned up with very little pressure. For 
convenience in placing the paper in position, the easel may be painted 
white and ruled with heavy black lines for the different sizes of en- 
largement or in one half inch squares numbered in large figures each 
way from the center on a vertical and a horizontal line. A further 
refinement consists in provisions for raising or lowering the easel and 
for sliding it to the right or left. By this means the portion of the 
negative used for projection may be brought within the limits of the 
area marked on the easel for various sized enlargements. 

Mr. E. J. Wall coats the easel with a mixture of the following com- 
position which does not dry but remains tacky so that the sheet of 
paper placed in position and rubbed down will be held in place for as 
long as required, after which it may be stripped off without difficulty : 


ee 2a klsy sang oo ce da eV Vales 407 gr. 53 gm. 
RE RMEAMM tel Fo5 2c Kh, ade a SY aba He Vad 407 er. 53 gm. 
a Ni cg einige w Agia sow aig cig ¥ wih we ME 10%, 65 cc. 
Pee Ome) AGM se ee Pet ee we Che 8 er. I gm. 


Pc thy ec a) ea i a 16 oz. 1000 Cc. 


Any grade of cooking gelatine may be used. It is first soaked for 


432 PHOTOGRAPHY 


Y% hour in about 34 of the total volume of water to which has been 
added the syrup and glycerine, after which it is melted in a water bath 
by heating to 50° C. (120° F.). Dissolve the alum in an ounce of 
water. Then make the bulk of the solution up to 15 ounces, add the 
alum solution and strain through linen. Allow 65 cc. of this mixture 
to every 100 square centimeters of the easel (about I oz. to each 100 
square inches). The mixture sets in about 24 hours and the easel 
is then ready for use.* 

Perhaps an even more convenient method consists in using a large 
printing frame. Fig. 213 shows a commercial easel designed for use 


3 


Fic. 213. Ingento Enlarging Easel for Use with Printing Frame 


with printing frames and provided with guides in order that the frame 
may be returned to the same position when loaded with the sensitive 
paper as it occupied when focusing. For commercial use where 
large numbers of prints must be made rapidly with a projector of the 
horizontal type an easel such as the “‘ Westminster,” ? illustrated in 
Fig. 214, is very convenient. The easel itself is swung to the hori- 
zontal position for inserting the paper which is fastened in place by 
clamping over it the hinged sheet of glass, after which it is swung to 
the vertical position for the exposure. 

With projection apparatus of the vertical pattern the easel becomes 
a very simple affair. In this case a flat surface of sufficiently large 
dimensions with a sheet of clean glass free from flaws or two bar 
weights sufficiently long to keep the sheet of paper flat during ex- 

1 Amer. Phot., 1923, p. 717. } 


2 Made by Westminster Photographic Exchange, Ltd., 61 Piccadilly, London, 
W. C., England. 


PROJECTION PRINTING 433 


posure constitute all the fixtures necessary for speedy and efficient 
working. | 

Whatever the form the easel takes means must be provided for 
altering the distance between it and the projecting lantern and in such 


Fic. 214. Westminister Enlarging Easel 


a way that the parallelism between the plate and the easel may not be 
disturbed. For this purpose a grooved track may be made or mark- 
ers may be placed on the floor to indicate the position of the easel for 
different degrees of enlargement. 

The Negative for Projection Printing.—It is difficult to give any 
precise definition of the proper type of negative for projection printing 
since so much depends upon factors for which no definite numerical 
expression is available. Of primary importance is absolute freedom 
from physical defects of any kind such as scratches, pin-holes and 
spots of all kinds, as they are enlarged along with the rest of the nega- 
tive and become unpleasantly conspicuous in the finished print. While 
much may be done towards removing such defects by appropriate 
handwork, such work requires to be done very carefully as imperfec- 
tions which would not be seen on a contact print are only too prominent 
when enlarged. 

Hunter and Driffield were the first to call attention to the fact that 
positives obtained by projection possess more contrast than contact 
prints from the same negative and on the same printing material.’ 
Seven years later Chapman Jones * investigated the scattering of light 
by the photographic plate and in 1909 Andre Callier in a paper before 


Puoss G. £., 1ngt, 10, 98: 
4 Phot. J., 1808, p. 102. 


434 PHOTOGRAPHY 


the Royal Photographic Society of Great Britain® showed precisely 
how this was responsible for the difference in contrast between posi- 
tives made, by projection and those made by contact printing from the 
same negative and on identical printing media. He says: 

“In projection there. is, of course, a scattering of the light trans- 


mitted by the negative (Fig. 215). The ray SN coming from the. 


Fic. 215. Scatter of Light by Negatives. (Callier) 


light source S is scattered in passing through the negative NV, and only 


a part of the light coming from the negative can enter the lens. As 


in the transparent parts of the negative the loss by scatter is nearly 
zero (owing to the relative absence of reduced silver), it follows that 
the contrast between the non-scattering parts of no density and the 
scattering parts of high density will be increased by the scatter. In 
contact printing this scattered light is not lost, and consequently the 
contrast is much less than in the case of projection.” 

It follows, therefore, that negatives for projection printing require 
less contrast than those intended solely for contact printing. Owing 
to the intervention of certain factors, for which numerical expression 


is unavailable, no satisfactory means exist for determining the differ- — 


ence in contrast which should exist in negatives for projection print- 
ing and those destined for contact printing. Here, as in numerous 
other cases in practical photography, experience is the only safe guide. 
Fortunately, however, the differences among printing media and the 
opportunities for control in the operation of printing are such as to 
largely remove this disability so that it is, to a certain extent, possible 
to secure from any average negative an enlargement with virtually the 
same gradation as a contact print. 

In general, however, thin negatives, soft rather than hard, free from 
fog and from physical defects of all kinds, as well as any undue 
granularity, are best for enlarging. Recent researches have shown 
that there is virtually little or no difference among the common de- 
veloping agents with respect to the granularity of the image. The use 


5 Phot. J., 1909, 49, 200; Zeit. wiss. Phot., 1909, 7, 257. 


PROJECTION PRINTING 435 


of any particular developing agent is therefore not so important as the 
avoidance of high temperature in developing, fixing and washing: in 
prolonged drying in hot, humid air and in under exposure. All of 
these things tend to increase the granularity of the image and are to 
be avoided with negatives destined for projection printing. 

The Technique of Projection Printing.—Assuming the apparatus 
to be in order and everything ready for use let us consider briefly the 
technique of projection printing with different forms of apparatus. 
With daylight or with completely diffused light sources such as the 
Cooper-Hewitt mercury vapor lamp, reflected light or incandescent 
lights with reflectors, the operations are simple indeed. The light is 
first turned on and then the negative inserted in the carrier with its 
face towards the easel. The projected image is then focused roughly 
on the easel in order to determine the degree of enlargement. If this 
is satisfactory all that remains is to focus sharply, cover the lens, or 
turn off the light, place the sensitive paper in position and expose. 
However, if the projected image is larger or smaller than desired the 
easel must be moved nearer to, or farther from, the lens, as the case 
may be, until it is seen that the projected image is approximately the 
size desired, after which the image is sharply focused and the ex- 
posure made. 

These distances from the lens to the easel and from the lens to the 
negative are conjugate distances and may be readily calculated for any 
given set of conditions (Chapter III, page 68). The following 
table, however, will show the conjugate distances for all ordinary de- 
grees of enlargement and for lenses of the usual focal lengths. By 
“degree of enlargement” is meant linear enlargement. Thus from 
4 x 5 to 8 x 10 is two times enlargement, not four times. 

When condensing lenses are employed the operations are not so few 
in number or so simple. In this case the negative should be inserted 
in the carrier and the image roughly focused to the desired size. The 
negative carrier should then be removed and the light source adjusted 
to secure an evenly illuminated field of maximum intensity. These ad- 
justments have been noticed already. on page 427 of this chapter. 
The light source having been adjusted so as to obtain a uniformly 
illuminated field, the negative carrier is again inserted and the image 
accurately focused after which the exposure may be made. | 

Then the paper may be pinned in position and where the easel is not 
provided with means for altering its position in order that the pro- 
jected image may be brought within certain previously marked lines, 


ges Bae, ae 
s 
= 


436 PHOTOGRAPHY 


TABLE FOR CALCULATING DISTANCES IN ENLARGING OR REDUCING 


From The British Journal Photographic Almanac 


Focus of Lens Times of Enlargement and Reduction 
Inches 1 Inch | 2 Inches | 3 Inches | 4 Inches | 5 Inches | 6 Inches | 7 Inches | 8 Inches 
4 6 8 10 I2 14 i 
2s cA Wh tae 4 3 2% 24% 27/5 244 2*/; 24 
5 71% 10 12% | @I5 17% 20 22% 
PLS Spee b Selig ear 5 334 3% 3k 4 2/19 26/7 2/16 
6 9 12 15 18 21 24 27 
aR aap 6 4% 4 334 33/s 3% 35/7 3% 
“4 10% 4 17% 21 24% | 28 31% 
a Aa ee 7 5% 4% 434 41/5 4" ie 4 3°/10 
8 12 16 20 24 28 3 3 
6 Pee 8 6 5% 5 44/s 4% 44); 4% 
9 13% 18 22% 27 31% 3 404 
APS: Soto 9 6% 6 53/5 57/s 5% 51/1 5/16 
10 15 20 5 30 30 4 45 
Rios eras 10 714 6% 64 6 5°/s 55/2 5% 
II 164% | 22 27% 3 3844 | 44 49% 
546... uae II 84 74% 64/s 61% 6/12 62/7 63/16 
ra 18 24 3 42 48 54 
O35 een: 12 9 8 7% 7/5 7 65/7 634 
14 21 28 35 42 49 56 63 
0 Nn ee 14 10% 9% 834 82/; 81/6 8 7% 
16 24 3 40 48 56 64 a2 
8) A ee 16 12 1024 | 10 93/5 9% 91/7 9 
18 2 36 45 2 81 
Renee ee 18 13% 12 1% 104/5 10% 10?/7 10% 


The object of this table is to enable any manipulator who is about to enlarge 
(or reduce) a copy any given number of times to do so without troublesome 
calculation. It is assumed that the photographer knows exactly what the focus 
of his lens is, and that he is able to measure accurately from its optical center. 
The use of the table will be seen from the following illustration: A photog- 
rapher has a carte to enlarge to four times its size, and the lens he intends em- 
ploying is one of 6 inches equivalent focus. He must therefore look for 4 on 
the upper horizontal line and for 6 on the first vertical column and carry his 
eye to where these two join, which will be 30-714. The greater of these is the 
distance the sensitive plate must be from the center of the lens; and the lesser, 
the distance of the picture to be copied. To reduce a picture any given number | 


PROJECTION PRINTING 437 


of times, the same method must be followed; but in this case the greater num- 
ber will represent the distance between the lens and the picture to be copied, the 
latter that between the lens and the sensitive plate. This explanation will be 
sufficient for every case of enlargement or reduction. 

If the focus of the lens be 12 inches, as this number is not in the column of 
focal lengths, look out for 6 in this column and multiply by 2, and so on with 
any other numbers. 


it is preferable to cover the lens with a lens cap containing an orange 
light-filter transmitting rays to which the paper is insensitive. This 
colored screen may be prepared by soaking an unexposed, fixed-out 
and thoroughly washed plate in tartrazin, naphthol, yellow S, orange G 
or ammonium picrate, which should be used in saturated solutions and 
the plate immersed in the dye bath for about 15 minutes, then rinsed 
and dried. However, where lateral or up-and-down movement of the 
easel is possible it is perhaps preferable to line the board in squares as 
previously suggested and center the same with respect to the projected 
image when focusing. Then the light may be cut off entirely and the 
paper placed in position by the aid of the numbered squares. We 
have already shown on page 431 methods which may be conveniently 
employed. 

With automatic focusing apparatus in which the image is always in 
focus regardless of the degree of enlargement the process becomes as 
simple as contact printing. In this case one has only to adjust the 
distance between the projection apparatus and the easel to secure the 
size of image desired, after which the paper may be placed in position 
and the exposure made. 

Focusing.—There will be no difficulty in focusing as a rule; how- 
ever, with dense or fogged negatives and at high degrees of enlarge- 
ment some trouble may be experienced occasionally. In such a case 
it is well to take an old negative which is quite dense and make a few 
ragged scratches on it with any sharp-pointed instrument. This may 
be inserted in the negative carrier in place of the negative and can be 
focused sharply with ease, after which it is removed and the negative 
reinserted. Where the exact nodal plane of the projecting lens is 
known it is possible to construct a focusing scale for the lens and easel 
using as a basis the distances given in the table on page 436. It is not — 
often, however, that the positions of the nodes are known and in this 
case the method indicated by Mr. A. Lockett may be usefully em- 
ployed. All that is necessary to provide any enlarging lantern with 
an accurate focusing scale, by this method, is the precise determina- 


6 Brit. J. Phot., 1924, 71, 171. 


438 PHOTOGRAPHY 


tions of the positions at two different degrees of enlargement, say 3 
and 4 times linear, marking the position of the lens standard on the 
base of the camera for each degree of enlargement. From these two 
points we can calculate the position for any other degree of enlarge- 
ment. Since the marks 3 and 4 on the baseboard of the camera indi- 
cate the actual tested extensions for that degree of enlargement, when 
set at position 3 the conjugate focus must be F + (F/3) while at 4 it 
is F+-(F/4). Eliminating F which occurs in both, the distance 3 to 
4 is equal to %4—%, or 1% of the focal length of the objective, so that 
the distance 4-6 on the scale equals the distance 4-3. In a like man- 
ner we find that 

Distance 3-4 ==distance 4-6, 

Distance 3-6 = distance 3-2, 

Distance 2-3 ==distance 2-14, 

Distance 3-114 = distance 1%4-1, 

Distance 6-8 distance 6-4. 


Accordingly the scale of positions on the baseboard of the camera will 
appear as in Fig. 216. Projection printing with a lantern thus fitted 


Fic. 216. Graduating Focusing Scale for Enlarging. (Lockett) 


is only slightly less rapid and convenient than with the more expensive 


automatic focusing apparatus, since nothing more is required than to - 
set the position of lens to scale according to the degree of enlargement | 


required. 

Determining Exposures in Projection Printing.—The usual method 
of determining the proper exposure is by trial with test strips and this 
is the only certain method in practice. Several methods of determin- 
ing the proper exposure have been devised by various writers but on 
the whole they all demand more work and time than the average worker 
is willing to spend and in practice the method of exposing test strips 
is almost always used. 

Edward S. King ‘* and Rev. F. C. Lambert * have described methods 


7 King, Brit. J. Phot., 1906, 53, 188. 
8 Lambert, Amat. Phot., 1921, p. 161. 


PROJECTION PRINTING 439 


in which the image thrown upon the easel is examined by a candle and 
the distance noted at which the candle must be placed in order to ob- 
literate all traces of the image. This distance in inches is then squared 
and the result multiplied by a correction factor which depends upon 
the aperture, the paper, etc., and is found by trial for any given set of 
conditions. 

In practice the writer has found extinction photometers of the Hyde 
exposure meter and Ica Diaphot type useful and convenient as indica- 
tors of the approximate exposure. ‘The projected image is examined 
through the instrument and the black-glass wedge turned until the de- 
tails in the highlights are just visible. <A little experience will enable 
the proper point to be reached quite readily. The exposure indicated 
on the meter is then multiplied by a correction factor which depends 
upon the speed of the paper, the stop in use, and the color of the nega- 
tive.® 

Relative Exposure, Scale and Aperture in Enlarging or Reduc- 
tion.—When the best exposure has been found for a given negative 
at a certain size of enlargement, the time of exposure at any other 
degree of enlargement or reduction is easily found, provided condensers 
are not used. Perhaps the most convenient method of making such 
calculations is by the charts described by Capt. S. M. Collins." 


c!S a 6 50 
40 C IS5469e a 4+ 
10 be 
8 30 10 bie 6 
g +30 
6 w £09 6 ers Bye 
5y 5 2 PAS el me gi tse 
40 = ape S4t) 5. x 5 15 ae 
S = = = 12) = 
_— ms) 1 eanw ' TS oO aon 
é £ 10 Ob £ P2015 = 
27 8 8 6 
: : 241 30k 
3s 6 Ip£5 40+10 
z 5 4 50 
4 L535 60415 
| 3.5 


In the general problem of exposure there are 3 factors which may 
all vary; these are: the size of the picture, the aperture of the lens, the 
duration of the exposure. Any two being known the other can be 


9 For a very complete system of exposure calculation. based upon measure- 
ment of the highest density of the negative in a wedge photometer, see article 
by J. M. Sellors in the British Journal of Photography, 1923, 70, 349. 

10 Brit, J. Phot., 1923, 70, 31. 


440 PHOTOGRAPHY 


found. The following scales provide a means of making these calcula- 
tions. From Chart I is read the alternation in the F/number required 
when varying the size of reproduction to obtain the same exposure. 
In Chart II, C and D show the mutual relation of size and exposure if 
the same F/number is retained: and scales E and D connect the 
F/number and the exposure when it is necessary to change either 
without varying the size. 

To find the proper diaphragm when it is desired to alter the size of 
reproduction, but not the exposure, using Chart I run the edge of a 
ruler from the size of the image to the aperture used. Mark the posi- 
tion at which the ruler cuts the index line. From this position run 
the ruler to the new size of reproduction. The other end cuts the 
aperture scale at the proper aperture to employ. 


To determine the time of exposure with the same aperture for an- — 


other degree of reproduction we make use of scales C and D of Chart 
II. The straight edge is applied to the scales so that it cuts scale C at 
the size of image for which the proper exposure is known and scale 
D the time of exposure. Mark on the index line the point of intersec- 
tion as before and join this point to the corresponding size of image on 
scale C. The other end of the ruler will then cut the scale D at the 
proper time of exposure for the new size of reproduction. 

The aperture required for any given exposure when that for a given 
exposure is known may be calculated from scales D and E. With the 
edge of a ruler join the aperture on scale E with the corresponding 
exposure on D. From the point of intersection with the index line 
transfer the ruler so that it passes through the exposure required on 
scale D. Then the other end cuts scale E at the proper aperture to 
use. 

Introducing Clouds in Enlargements.—In the case of landscape 
negatives with a bald-headed sky, it is often possible to improve the 
pictorial effect considerably by printing in clouds from another nega- 
tive. While there are several ways of doing this, the method to be 
described is as satisfactory as any and is perhaps the most generally 
useful. The image of the landscape negative is first focused on a sheet 
of thin cardboard placed on the easel. On this sheet of cardboard the 
outline of the skyline is traced with a soft pencil. The cardboard is 


now removed and cut along the pencil line. The upper piece will’ 


serve to mask the sky portion while making the exposure for the land- 
scape while the lower portion will serve to mask the landscape while 
the cloud is being printed. 


: 


“ 
“orn . : 
a hr Pa - * ; A 
See ee ee ee eee 


PROJECTION PRINTING 44] 


This much done, the bromide paper is placed in position and the 
proper exposure given for the landscape portion masking the sky por- 
tion by the sheet of cardboard. It is not often that the sky portion re- 
quires any masking, but when it does the appropriate mask is held 
close in front of the bromide paper and kept in slight up-and-down 
movement with the cut-out outline in close register with the image. 

Now replace the orange cap on the lens and trace lightly on the 
bromide paper with a soft lead pencil the outline of the sky. Then, 
without moving the paper, replace the landscape negative by the cloud 
negative and adjust the latter so as to secure the clouds in the desired 
position. Next bring the other mask in position so as to cover the 
landscape portion and make the exposure for the clouds, keeping the 
mask moving up and down as before. 

If the proper times of exposure for the two negatives have been 
accurately determined by exposing pairs of test strips and developing 
the same together for the same time, this procedure should result in a 
satisfactory result. 

Of the esthetic factors in the combination of landscape and sky 
from separate negatives, it is not within the province of this work to 
speak. The worker’s sense of the esthetic and his knowledge of na- 
ture must ever be on guard in combination printing in order that the 
result be true to nature and satisfactory to the artistic sense. 

Enlarged Negatives.—Where a large number of prints are required 
it is sometimes more convenient to make an enlarged negative and 
print from it by contact, while certain printing processes as gum, 
carbon, and oil require an enlarged negative if prints larger than the 
original negative are desired. 

There are two practical methods: In the first a contact positive is 
made from the original negative using either the carbon process or a 
slow dry plate and from this positive the enlarged negative is made in 
the ordinary way. The second method consists in making an enlarged 
positive of the size required and from this making the negative by con- 
tact printing. Other methods involving the reversal of the image have 
been advised but as they are hardly suitable for practical use they will 
not be discussed. 

Sensitive Materials—The sensitive materials used are an important 
factor in securing satisfactory results. Most inexperienced workers 
make the mistake of selecting for this work plates of the transparency 
or lantern slide type. While admirably suited to positives for visual 
examination, such plates are not well adapted for making either the 


442 PHOTOGRAPHY 


intermediate positive or the final negative. Far better results are to be 
had from the use of a medium speed, clean-working plate of normal 
contrast. The writer has used successfully Eastman Commercial 
Film, Imperial Fine Grain Ordinary and Cramer Slow Iso plates. 
The class of plates of which the above are typical representatives are 
rather faster than ordinary enlarging materials, especially the East- 
man film, which is a comparatively fast yet clean-working emulsion. 
They consequently demand greater caution in handling and exposure, 
but give better gradation than the contrast-working transparency plates 
which tend to produce blocked highlights and clear shadows, causing 
a loss of detail at both ends of the scale, together with excessive con- 
trast as a whole. Extra rapid plates are of course somewhat more 
difficult to handle and do not give contrast or density quite so readily 
as those of slower speed. This property may of course become an 
advantage in dealing with contrast originals, for which plates of the 
rapid class may be used with advantage, just as transparency plates 
may be serviceable at times with very weak originals, but on the whole 
it is preferable to select a clean-working, fine-grained plate with an 
H. and D. speed ranging from 100 to 150, as the contrast of the final 


result may be controlled to the extent usually required by alterations 


in the time of development. 

Exposure.—The conditions of exposure will naturally vary with the 
materials chosen and the equipment of the individual worker. Care 
must be taken during all operations to see that both the negative and 
the sensitive materials are completely free from dust, otherwise there 
is likely to be a fine crop of small transparent spots which completely 
ruin the result. It is well to call attention to the fact that perfect 
contact between the two surfaces is essential. The slightest want of 
contact which may not be observable in the small positive will become 
serious when enlarged, particularly if the degree of enlargement is 


considerable. For this reason the light amateur printing frames 


should not be used for this purpose, but rather the heavy professional 
frames which are equipped with much stronger springs. It is also of 
equal importance that the focus be accurate when enlarging. The 
simplest way of ensuring accurate focus is to replace the usual easel 
with one consisting of a removable sheet of ground-glass on which the 
image can be focused by transmitted, rather than reflected light. The 
use of a ruled test plate in the negative carrier is also to be advised. 

The time of exposure will naturally vary with conditions and must 
be determined by test. For this purpose the first plate should be ex- 


PROJECTION PRINTING 443 


posed in strips, giving a range of exposures from which the proper 
time of exposure may be determined after development. The proper 
exposure is that which is just sufficient to penetrate the deepest de- 
posits and produce a deposit on the sensitive material. No part— 
excepting perhaps a small point—of the intermediate positive should 
be clear glass. Even the highest highlight should show a slight de- 
posit. Nor should the deepest shadows be of any great density. 
What is required is a soft, almost flat-appearing positive of full detail 
and delicate gradation. Under exposure is to be avoided and par- 
ticularly worthy of serious attention is that slight under exposure 
which tends to give a brilliant, clean-cut result. Invariably this 
bright, snappy, clean-cut result, for which the inexperienced worker 
is quite enthusiastic, is the result of slight under exposure and is ac- 
companied with a loss of gradation at both ends of the scale, but more 
particularly in the highlights. The strip which has received from two 
to four times the exposure of the snappy-appearing strip (which 
suggests a good lantern slide) is a better indicator of the proper ex- 
posure than the brilliant strip. 

Development.—To accurately reproduce the original negative both 
the intermediate positive and the negative must be developed to a de- 
gree of contrast equal to that of the original negative, or what is 
termed technically a gamma of unity. If it is desired to lessen the 
contrast of the original, the time of development of either the positive 
or the negative, or both together, may be shortened so that each is 
developed to a stage of contrast less than unity. This is a matter 
far more easily accomplished with exactness by time development 
than by either inspection or factorial methods. 

The Watkins thermo system of development is perfectly adapted 
to the development of both the intermediate positive and the final en- 
larged negatives. The thermo system, however, is calculated for a 
degree of contrast of less than unity (.9), hence if it is desired to 
secure an accurate reproduction of the contrast of the original nega- 
tive a longer time of development than that indicated by the tables is 
required. While I have not calculated its mathematical accuracy, I 
have secured results sufficiently exact for all practical purposes by 
developing the positive as directed by the tables, but classifying the 
plate or film used for the negative one class higher than listed on the 
table of developing speeds. This opposition of effect, while perhaps 
not mathematically exact, gives approximately the same degree of 


444 PHOTOGRAPHY 


contrast as the original negative. To reduce the contrast of the final 
negative both the positive and the negative may be developed as in- 
dicated by the tables, or in the same way, to increase contrast both 
may be developed as if listed one class higher. In either case, the 
main point is that one is working under standardized conditions, which 
enable the source of trouble to be located and the necessary changes 
in procedure made in a calculable way. 


GENERAL REFERENCE WorKS 


BayLey—Photographic Enlarging, 1923. 

FRAPRIE—How to Make Enlargements. 

SMITH—Fnlargements—Their Production and Finish. 
SNopGRASS—The Science and Practice of Photographic Printing, 1923. 


CHARTER! XIX 
LANTERN SLIDES AND TRANSPARENCIES 


The Lantern Slide and Its Uses.—The lantern slide is simply a 
print on glass measuring 3% x 4 in this country, 314 x 3% in Eng- 
land and 9 x 9 or 9 x 12 in France and other countries using the 
metric system. Slides are useful to the lecturer, instructor, enter- 
tainer, and advertiser. The statement has often been made that pic- 
tures tell a story quicker and better than words, and we may add that 
pictures give a clearer impression than can be secured from a word 
description. Just as the appeal to the eye is more effective than the 
appeal to the ear, so is pictorial representation more effective than 
description. Thus a written description, however detailed and ac- 
curate, can never quite bring home to us the characteristics of Gothic 
architecture as a collection of photographs of the famous cathedral of 
Rheims. 

The Negative.—In no branch of photography is better technical 
work required than in making lantern slides and the proper place to 
begin is with the negative. The clever worker may scrape and make 
numerous alterations on his negative in such a way that no one will be 
able to tell the difference in the print, but practically no handwork may 
be done on a negative from which a slide is to be made, for every 
touch is magnified from fifty to a hundred times and becomes painfully 
evident on the screen. Therefore if one is making negatives which 
may be used for slides it is important to choose a plate of fine grain 
and reasonably free from mechanical defects. The interior of the 
camera and the plate holders should be kept free of dust and the plates 
carefully dusted before loading the holders. Fixing and developing 
solutions should be fresh and filtered and'‘it is better to fix in an’ up- 
right tank rather than a tray. After washing, each negative should 
be cleaned with absorbent cotton to remove adhering sediment and the 
negative placed in a dust-free place to dry. Too much trouble can 
hardly be taken in securing the finest quality negative as practically 
nothing can be done to remedy faults in technique, except, of course, 
reduction or intensification. 

Lantern Plates.—Nearly every plate manufacturer makes at least 

a 445 


446 PHOTOGRAPHY 


two varieties of plates for lantern slides. We may divide the com= 
mercial plates into three classes : 3 


1. Fast plates for reduction in the camera. 
2. Slow, “ gaslight”’ plates for contact printing. 
3. Plates of extreme contrast or made especially for warm tones. 


Representative brands of the first class are the lantern plates of 
Cramer, Hammer, Eastman, Central, Agfa, Hauff, Gevaert, Ilford, 
Barnet, Illingworth, Imperial and Wellington. 

Slow lantern plates, handled in almost the same manner as “ gas- 
light” paper, are almost entirely of foreign manufacture. Prominent 
among plates of this type are the Wellington S. C. P., Imperial Gas- 
light, Paget Gravura, Ulingworth Slogas lantern plate, and Gevaert. 

Various tones may be secured on most of the lantern plates named 
above by proper manipulation but there are a few plates which are 
made especially for the production of warm-toned slides, namely, ks 
Ilford Alpha, Central Sepiatone and Grieshaber’s Varieta. 

Printing Frame for Contact Printing.—The beginner is advised to 
start by selecting negatives from which slides may be made by contact 
printing and, after he has mastered this method and can make a good 
slide from any reasonable negative, he may take up reduction. For 


Fic. 217. F and S Lantern Slide Printing Frame 


contact printing the negative must not be larger than 3 x 334 inches 
but it often happens that only an area of these dimensions is really 
wanted from a larger negative and contact printing is then possible. 
While an ordinary frame may be used it is better to either purchase a 


LANTERN SLIDES AND TRANSPARENCIES 447 


special lantern slide frame such as the one illustrated (Fig. 217) or to 
make one from the following description. 

Select a frame two or three sizes larger than any of the negatives 
from which slides are to be made, say 8x1o for 5x7. In this is 
placed a piece of plain glass and over it a piece of opaque paper in the 
center of which has been cut an opening 3.x 4 inches. Instead of the 
usual hinged back one of a piece of flat wood, its under side covered 
with felt, is fitted in. This has an opening in the center measuring 
3%x4. A little door of the same dimensions is hinged to one side of 
the larger back and one of the springs attached to fasten the door and 
secure contact between the negative and the lantern plate. To use the 
entire back is taken out and the negative inserted, the desired portion 
being placed exactly over the opening in the black paper. The back 
is now replaced and fastened down. The small door may then be 
opened, the lantern plate inserted and the exposure made. Any 
number of slides may thus be made from the same portion of the 
negative without readjustment. 

Exposing.—For exposing use any artificial light, preferably elec- 
tricity. The bulb should be frosted or an image of the filament is 
liable to fall on the frame. A board should be marked off so that the 
distance from the frame to the light may always be the same, and 
variations in the strength of the light eliminated. It is impossible to 
give any idea as to the length of exposure since lights differ and no 
two brands of plates have the same speed. The best plan is to make 
a series of trial exposures for different times on the same plate and 
from this series pick the one giving the best results. To do this hold 
a card in front of the frame and uncover an inch of the plate for say 
two seconds, then shift the card so as to expose another inch and give 
another two seconds and so on until the whole plate has been exposed 
and we have four strips the exposures of which are 2, 4, 8 and 16 
seconds. In order to make the strips equal and regular the positions 
of the card may be marked on the outside of the frame. After de- 
velopment and fixation the plate may be examined and the exposure 
giving the best result readily determined. Before treating develop- 
ment, however, we will discuss the advantages and methods of making 
slides by reduction. 

Printing by Projection—The writer firmly believes reduction to 
be superior to contact printing. The definition is better, there is less 
danger to the negative, or the lantern plate, and any shading to lighten 
or darken parts of the slide is more easily done. There is also the 


448 PHOTOGRAPHY: 


great advantage of being able to make a slide from a negative of any 
size when either wet or dry. This last feature is particularly desirable 
when an advertising slide is wanted in a hurry. 
Slide making by reduction is simply rephotographing the original 
negative on the lantern plate. Special cameras are available for this 
purpose (Fig. 218) having at one end a set of nested kits for holding 


Fic. 218. Century Lantern Slide Camera for Reduction 


the negative and at the other an adjustable back taking a plate holder 
of lantern slide size, the lens being fixed in the central compartment. 
Except for slide making in quantity, their expense is not justified. 
Another method consists in blocking out a window in the same way 
as described on page 418 in the chapter on projection printing and 
placing the camera on a sliding track facing the negative as shown in 
Fig. 219. For slide making in quantity, however, artificial light of 
some kind is far more satisfactory than daylight owing to its uniform- 


Fic. 219. Slide Making by Reduction Using Daylight 


ity. If desired, artificial light may be used in a similar way, condens- 
ing lenses or reflectors being used to secure uniform illumination. 

It is more convenient to use for this purpose a camera which focuses 
from the rear. With a camera focusing from the back it is possible 


LANTERN SLIDES AND TRANSPARENCIES 449 


to set the lens at the conjugate focus with respect to the original and 
the degree of reduction required and focus without disturbing this 
relation. This is not possible with a camera which must be focused 
from the front as any movement of the lens to secure accurate focus 
naturally disturbs both conjugates. 

Those provided with a good enlarging lantern may use it for reduc- 
tion by using a supplementary lens over the regular lens to shorten the 
focal length, or better by building an extension cone sufficient to pro- 
vide the amount of bellows extension required. The length of. the 
cone will depend upon the focal length of the lens, the degree of reduc- 
tion and on the length of the bellows fitted to the machine. The tables 
of focal distances given in the chapter on projection printing will be 
of assistance in determining the length of the extension cone. 

The most satisfactory method which we have seen for holding the 
plate in position for the exposure is that described by Mr. D. Charles 
in the British Journal of Photography* and illustrated in Fig. 220. 
The details of construction will be apparent upon a thorough examina- 
tion of the illustration. In the writer’s opinion, sharper focus may be 
obtained when focusing is done on a ground glass rather than by re- 


Fic. 220. Device for Holding Lantern Plate in Position When Using Enlarger 
for Lantern Slide Making by Reduction. (Charles) . 


flected light. When a ground-glass screen is used methods of Parallax 
focusing as described on page 556 in the chapter on Copying 8 be 
employed to advantage. 

As in the case of contact printing, exposures are best determined by 
test plates exposed in sections. Calculations on the order of those 
described previously in the chapter on Projection Printing, however, 
may be useful as a first approximation. 

Developers.—A developer suitable for lantern slides should be non- 


1 Brit. J. Phot., 1922, 69, 23 


450 PHOTOGRAPHY 


staining, free from fog, and produce an image of good color, fine 
grain and comparatively high contrast. No developing agent has ever 
surpassed the old ferrous oxalate in meeting these requirements and it 
is, therefore, the developer par-excellence for lantern slides and trans- 
parencies. It is now seldom used, however, having been replaced by 
the more convenient and more energetic organic developing agents. 
Nevertheless we give one formula: 


A.. Ferrous sulphates 2) ),... ss 32s wae eee 5%4 oz. 330 gm. 
Water “to makes 23 2: v Ciiee o 16° As OF 1000 cc. 
Sulpinric® acid: sh 2000. 5 a5. sah ane 7 uh ae Toc, 


Only the highest grade ferrous sulphate should be used and this 
should be fresh. The solution oxidizes rapidly and it is well therefore 
to prepare no more than will be used in a day or so. 


B. Potassiuti oxalate: ix. sfcu ane Wk 5%4 oz. 330 gm. 
Hot water: to make... 50.00 235 eek ee 16. Oz, 1000 cc. 


For use take one part of A to four parts of B. Add A to B and 
not vice-versa or an insoluble iron salt will be formed. 

Next to ferrous oxalate one of the best developing agents for trans- 
parencies is glycin. The following formula is suitable (Hubl) : 


Sodium sulphite (dry) :>...s ake suas ee 14 oz. 125 gm. 
Warm water to make..5.4. 02.02. ob: ae 4.0% 400 cc. 
Glycin ..... were t te ere ee tome 100 gm. 


Mix well and add gradually: 


Potassium carbonate... [alc cw.ctame vc one ste So Ge, 500 gm. 
Water to make... .... 05. 74 OZ. 750 cc. 


For use dilute with 8-12 parts of water. Slides must be well washed 
before placing in the fixing bath or stain will appear. | 
Perhaps the most generally used developer for lantern slides and 
transparencies is hydrochinon. It has the advantage of giving good 
contrast and satisfactory color but the quality of the image is not so 
good as glycin. The following formula is suitable Cea 


A. Hydrochinoti: 0.02. <4 vas +e eee eee ist 10 gm. 
Sodium euiphite: (dry) sia 22 ey ee 154 gr. 20 gm. 
Potassium: ferrocyanide;’,). 24s. 25a ee 922 gr. 120 gm. 
Water: to “make. «<i> ox anapeee as: ae 16.07; © 100g sae 

B. Caustic sodas. iv cays naan «eecn eae ee 384 er. 50 gm. 


Water: to “makes... oc 05S 2 As Po eee 16 oz. 1000 cc. 


LANTERN SLIDES AND TRANSPARENCIES 451 


For use take 10 parts of A to one of B. For general use with 
American plates, however, the developer as made up above may be 
further diluted with an equal part of water. This developer works 
with medium contrast. If a hydrochinon developer giving maximum 
contrast is required the hydrochinon-caustic soda formula given on 
page 305. 

For very fine grain images with the hydrochinon developer add 50- 
300 grams (385 grains 5% ounces) ammonium chloride to each 1000 
cc. (16 oz.) of the above developer. 

Other developers suitable for lantern slides are amidol and metol- 
hydrochinon. Pyro may be used, but excepting for warm-toned slides, 
is not so convenient, nor altogether as satisfactory, as the non-staining 
agents already noticed. 

Development.—The writer expresses a decided preference for the 
Watkins factorial method in developing lantern slides and trans- 
parencies. ‘The density of a slide should be determined solely by ex- 
posure, and development regulated so as to produce the degree of con- 
trast required. Once the proper factor has been found for the de- 
veloper and plate in use it is a simple matter to locate errors in ex- 
posure and development. The following table shows how to deter- 
mine the principal defects of exposure and development and to remedy 
the same. 


Fault ne Cause Remedy 
Too much density, Over exposure Give less but develop 
correct contrast. to same factor. 
Too little density, Under exposure Give more but develop 
correct contrast. to same factor. 

Too much contrast, Over development Develop to lower factor. 
proper density. Give same exposure. 
Too little contrast, Under development Develop to higher factor. 
proper density. Give same exposure. 


The proper factor can be found only by experiment. It is dependent 
upon the plate, the developer and the degree of contrast required. In 
all cases it is less than that required for negative development and as a 
rough guide a factor about °4 of the regular Watkins factor may be 
taken for the first trials. 

The proper safelight adds greatly to the accuracy in observing the 
first appearance of the image and in determining the course of develop- 
ment. For the very fast lantern plates an orange safelight, such as 
the Wratten Series 0, should be employed; for the slower gaslight 
plates the Series 00, a bright-yellow, is safe. 


452 PHOTOGRAPHY 


Fixing, Washing, Drying.—After development, rinse well in run- 
ning water and place in an acid fixing bath, the one given in the 
chapter on fixing being entirely suitable. It is important that the 
fixing bath be kept fresh and acid at all times, otherwise there is 
liable to be a slight stain on the slides, which, while not particularly 
noticeable when the slide is held in the hand, will injure the trans- 
parency and brilliancy of the highlights when projected. Wash for 
fifteen to twenty minutes in running water and on removing wipe 
the surface with a piece of wet absorbent cotton to remove adhering 
grit and dirt. The slides should then be placed in the rack to dry, 
leaving at least two inches between each plate in order to ensure a 
good circulation of air. The use of an electric fan is advisable 
where possible. It is very important, however, particularly where a 
fan-is used, that the atmosphere be free from dust and perhaps one of 
the best ways to prevent dust from settling on them is to place a 
piece of newspaper over the drying rack. Then, if the air is not 
filled with dust stirred up by cleaning or some similar operation, they 
will dry comparatively free from dust particles. 

Masking.—-This is a very important operation, particularly where 
the subjects are pictorial, as the composition of the finished picture 
is largely determined at this stage. Of course as much of this should 
be done in the process of reduction as possible in order to secure the 
largest possible picture on the slide plate. In nearly all cases the 
matts used for masking off the undesired portions of the picture 


should have square corners. It is only occasionally that circular and — 


oval shapes may be usefully employed. Ready cut matts are fur- 
nished commercially in a wide variety of shapes and sizes but with 
most of these the corners are rounded and except in some cases this is 
nearly always objectionable. Adjustable matts with which any de- 
sired size or shape may be obtained are far more generally useful, for, 
although they cannot be so quickly applied, there is no need to keep 
on hand a large number of various sized matts since the proper pro- 
portions and size for any requirement may be prepared from the 
-single block of adjustable matts. 

Spotting.—After the mask has been fixed to the slide with a little 
glue and has set under pressure until there is no danger of move- 
ment, the slide may be spotted. Spotting is employed to guide the 
operator in placing the slide in the lantern correctly. The rule to fol- 
low in placing the spot is to hold the slide as tt should appear on the 


LANTERN SLIDES AND TRANSPARENCIES 453 


screen and place the little square of gummed white paper in the lower 
left corner. The lantern operator places the slide in the lantern up- 
side down with the spot acting as a thumb mark. 

Binding.—This is the final operation and one at which the beginner 
is not usually successful. Not that it is a difficult operation, but 
there is a little knack to it which is readily acquired with experience. 
The easiest method is to employ four strips, although many experi- 
enced hands prefer the continuous strip method. To use the former, 
begin by cutting a number of strips of binding paper 3% and 4 inches 
in length. The former are used for binding the ends; the latter the 
sides of the slide. Then take a piece of absolutely clean cover glass 
(a cleaned-off slide plate) and place it against the masked side of the 
slide. The binding strip is then moistened and applied to the edge 
of the slide. First see that the strip of binding tape is placed evenly 
so that it will fold over regularly and uniformly on both sides of the 
slide. Proceed in a like manner with the other four sides, then place 
the slide under pressure for an hour or so. 

Advertising Slides.—One is often asked to produce a slide showing 
both printed matter and an illustration. If the two are copied on a 
plate suitable for the photograph, then it is impossible to obtain suf- 
ficient contrast in the legend, while on the other hand if a process 
plate is used to reproduce the legend correctly the photograph is 
excessively contrasty. There are two ways of overcoming this. One 
way is to make the copy on a plate of low speed which will give 
fair contrast so as to obtain the very best result for the photograph 
ignoring the legend. When the negative is dry, cover the photograph 
with any waterproof varnish and immerse the negative in a ferri- 
cyanide-hypo reducer until the lines of the legend are clear. Then 
after a thorough washing, intensify in Monkhoven’s intensifier to 
secure the proper contrast. When dry the negative may be printed in 
the ordinary way with good results. 

Another way consists in making one negative for the photographic 
portion and one for the line portion, using the appropriate sensitive 
material for each. The portion representing the photograph on the 
process negative is then blocked out by means of opaque while the 
legend is blocked out on the negative made for the illustration. 
Means of registration having been provided it is then a simple mat- 
ter to secure a slide of the proper quality by double printing, first for 
the illustration and then for the legend. 


454 PHOTOGRAPHY 


Toning of Lantern Slides by Restrained Development——Warm 
tones of black, brown, red, purple and sepia may be obtained by the 
use of a developer heavily restrained with soluble bromides and the 
colors so obtained are usually, in the writer’s opinion, superior to 
those obtained by processes of after-toning. The principle consists 


in the over exposure of the slide plate followed by development in — 


a developing solution highly restrained with potassium or ammonium 
bromide or a combination of the two. Not all plates produce satis- 
factory tones with such treatment and the best tones are secured on 
the plates advertised as warm-tone by the makers and on the slower 
brands of slide plates such as Standard Slow, Paget Gravura, Welling- 
ton S. C. P., etc. The following formula is recommended for the 
Wellington S. C. P. and when properly used will be found satisfactory 
for other makes of like nature: 2 


A, Metol-o, cscs es cigs: ation » pupae state, geet eee an 20 gr. 2. ‘gm: 
Hydrochinon: . $3 sc5s saeco oe 60 gr. 6 gm. 
Sodium sulphite {dry} .... 2: <3. 00. sues 3) 3gSOCats 35 gm. 
Sodium carbonate (dry)....55 70, es eee 350 er. 35 gm. 
Potassium. bromide.:. 4-4 (s405 sense 20 gr. 0.6 gm. 
Water to. makes 0. 2e.s as a 20 OZ. 1000 cc. 

B. Ammonium carbonate. 3145. ..5' ee eee I OZ. 100 «gm. 
Ammontum -bromidé..).5..... 4. sees eee eee { OZ. 100 gm. 
Water’ to ‘maker. oii... ae ee 10 Oz. 1000 cc. 


Without solution B the tone is blue-black. By increasing the ex- 
posure and adding correspondingly larger amounts of restrainer 
warm-brown, sepia, purple and red tones may be obtained. The fol- 
lowing table gives the approximate increase in exposure and the com- 
position of the developing solution for the various tones: 


Color Exposure x normal Developer 
Black 0 Fe i oe eee ee Normal At part, B part 
Warm-Black 9..0.4'vac vas Gee eee eee Normal x 1% A I part, B & part 
Browns. 5 eo) oA vial acelin eee ae Normal x 2 A 1 part, B &%& part 
VY AY TSO DIS © x5. sacs tooo wilco Normal x 3 A 1 part, B % part 
PUTO ious wecdinvaby . hoe Normal x 4 A I part, B &% part 
REG ae en oan bord. ee ee Normal x 6 A I part, B & part 


The appearance of the slide as it lies in the tray is not an accurate 
indication of its final color and the best results are secured when de- 
velopment is conducted factorially. The exposure determines the 
tone to a large extent and development must be regulated accordingly. 


LANTERN SLIDES AND TRANSPARENCIES 455 


The proper factor varies from three to five with the above formula. 
For the Wellington S. C. P. lantern plate a factor of three is recom- 
mended, while the writer has found five to be the best for some other 
brands of plates. 

Physical Development.—The advantages of physical development 
are: facility of obtaining soft slides from harsh negatives,’ the trans- 
parency of the shadows, the unique bluish-black tone and the ex- 
cellent results obtained by sulphide toning. The precautions to be 
observed are: 

1. Use only fresh plates. 

2. Give about double the exposure ordinarily demanded for regular 
developers. 

3. Keep all trays absolutely clean. 

The following is the formula of Dr. Mees: 


RE eh, cles Vy es Pek e eden we eens go gr. 5 gm. 
Ce tO) Ey ee SE oe Se ea ae eae Qo gr. 5 gm. 
Oy ESS EE | Re eae Riper Ze eC. 
MER OR RO se ak ilies cys var sev aseececs 20 OZ. 500 cc. 
ECR Sc hc he xy a Ws ss cee seas ce te I Oz. IO gm. 
CS LS See a IO Oz. 100 cc. 


To develop a slide 1 ounce of A is poured in a clean glass graduate 
and 50 minims of B added. The exposed slide is placed in the tray, 
the mixture poured on and kept in motion. During development the 
silver may be deposited ali over the plate but this can be removed by 
rubbing with wet cotton wool. As soon as the developer becomes 
brown it should be discarded. Fresh developer must be used for each 
batch of slides and the trays and graduates cleaned thoroughly each 
time before mixing in order to insure absolute cleanliness. Physical 
development is not a process for commercial use but is an interesting 
method for the amateur who desires unusual effects. 

Colors on Direct Development with Thiocarbamide.—The addition 
of thiocarbamide to a developing solution of metol and hydrochinon 
restrained with ammonium bromide for the purpose of obtaining a 
wide range of colors ranging from a delicate violet through red, blue, 
blue-black and black on lantern slides by direct development was first 
advised by Wratten and Wainwright, the English firm of plate makers, 
in 1909. The resultant image is of a very fine quality, with an unusual 
transparency in the lower tones which is obtainable in no other way, 
while the range of colors obtainable on slow lantern plates by modifi- 


456 PHOTOGRAPHY 


cation of the exposure, developer or temperature is unsurpassed by 
any method of toning by direct development. The process is a very 
difficult one and it is recommended that the student studiously avoid 
the same, not only until he has mastered ordinary slide making in 
black and white, but also until he is thoroughly familiar with the pro- 
duction of warm-tone slides by the methods previously described. So 
far, no definite information on the process can be given other than 
that which has been gained by methods of trial and error. Experi- 
ence alone can enable the worker to master the process. References 
to published work on the subject will be found in the bibliography at 
the end of the chapter. 

The Toning of Lantern Slides aiid: Transparencies.—With the ex- 
ception of the hot hypo-alum method, the methods of toning described 
in Chapter XX may be used for lantern slides as well as prints. Ex- 
cellent brown tones may be secured by the usual process of indirect 
sulphide toning ; the copper and uranium processes are also widely em- 
ployed. The toned image, however, leaves something to be desired 
as regards transparency, the tones produced by after-toning processes 
never equalling those produced by direct development in this respect. 

Probably the finest results in after toning are secured by dye-toning 
processes. Dye toning has found an extensive application in the mo- 
tion picture industry but does not seem to be widely employed for 
lantern slides. For further information on dye-toning methods the 
reader is referred to Lantern Slides—How to Make and Color Them 
obtainable free from the Eastman Kodak Company, Rochester, N. Y. 

Reduction and Intensification of Lantern Slides.—Neither the re- 
duction nor intensification of lantern slides and especially of warm- 
tone slides is to be recommended as a general rule. For reduction, a 
weak solution of ferricyanide-hypo may be used, but in the case of 
warm-tone slides only a slight clearing action should be attempted as 
substantial reduction alters the color. 

Where greater contrast is necessary it is preferable to intensify the 
negative rather than the slide. However, if for any reason this is un- 
desirable the slide itself may be intensified. The chromium intensifier 
is satisfactory and convenient for this purpose. For warm-tone 
slides, however, preference should be given to the following silver 
intensifier which has the advantage of not altering the color: 


LANTERN SLIDES AND TRANSPARENCIES 457 


a he RA a 88 gr. 8.8 gm. 
RO SOU Py os ease sate pa ve kee eles TDZ, Soe. 
Srttie acti... 3. Oo SHAPIRO Ee Varn OR ae ea 176 gr. 17.6 gm. 
a aD GY uns Dinly hig dle-G.8 ovale e 6 20 OZ. 1000 cc. 

(ENCE Sa I Oz. 50 gm. 
Be eR PTE oo j 5.m osha nin wed emi he aoe ee 20 OZ. 1000 CC. 


For use take A, 24 parts; B, 1-2 parts; distilled water, 24 parts. 

The intensifying solution must be prepared fresh for each slide. 
The dry slide is immersed in the intensifier for one to one and a half 
minutes until the required degree of intensification is secured. If it is 
allowed to act longer than a minute and a half it begins to work un- 
evenly and produces a blue deposit. Stains on trays, fingers, etc., 
from the intensifying solution may be removed by acidified perman- 
ganate solution or strong hypo and ferricyanide. When intensifica- 
tion is complete the plate is washed in running water for one minute, 
then immersed in an acid fixing bath for five minutes and finally 
washed in running water for one hour. 


GENERAL REFERENCE WorKS 


FrRAPRIE—How to Make Lantern Slides. 
Harris—Practical Slide Making. 
LAMBERT—Lantern Slide Making. 
Mercator—Die Diapositiverfahren. 


CHAPTER 


THE TONING OF DEVELOPED SILVER IMAGES 


Introduction.—For many subjects a color other than that of the 
cold neutral black of the ordinary developing papers is desirable and, 
when the color is selected properly with reference to the nature of the 
subject, adds considerably to the artistic effect. While of late there 
Las come into being a class of developing papers made for the special 
purpose of producing warm-black and brown-black tones by direct de- 
velopment and while some few papers may be made by restrained de- 
velopment to produce brown and sepia tones, in general, recourse must 
be had to toning processes for other colors than the usual black and for 
warm-black. There are almost innumerable variations in a large num- 
ber of toning processes producing results of varying quality and differ- 
ing greatly in adaptability to various emulsions. With some methods 
of toning, the colors are only slightly inferior to the corresponding 
images of prints produced by a pigment process such as, for example, 
carbon. With others, however, the results are not always such as 
please, much depending upon the suitability of the emulsion and the 
character of the print, while in some cases the process of toning itself 
is not above objection. A work of this nature is not the place for a 
comprehensive review of all of the many toning processes and their 
modifications. Representative formule with manipulative details of 
the more generally useful methods, however, are included. Those in- 
terested in the subject to a greater extent than it is possible to give to 
it in these pages are referred to the bibliography at the end of the 
chapter where will be found a fairly complete list of the principal 
works on the subject published during the last twenty years. 

The Sulphur Toning Processes—The Print—The most widely 
used processes of toning are those in which the metallic silver of the 
black image is converted into a colloidal silver sulphide. ‘The colors 
obtained by such treatment range from purplish-brown, through sepia 
and various shades of brown, to a disagreeable yellowish-brown. There 
are a number of processes which fall into this class and these may, for 
convenience in treatment, be divided into two divisions: (1) the in- 
direct processes in which the metallic silver is first bleached and then 

458 


TONING OF DEVELOPED SILVER IMAGES 459 


converted into silver sulphide by immersion in a bath of sodium, am- 
monium or barium sulphide; and (2) the direct method in which the 
conversion to silver sulphide is accomplished in a single solution. 
Certain differences exist in the nature and working of the two methods 
which may be more conveniently noticed when we consider the various 
processes separately. However, as the bearing of the black print on 
the final result is very nearly identical with all the processes of sulphur 
toning it is more convenient to consider this subject before proceeding 
to a discussion of the processes themselves. 

The color obtained upon toning depends to a certain extent upon 
the toning operation itself ; much more, however, on the exposure and 
development of the black print. If increasing times of development 
be taken and the exposure adjusted in each case so as to produce a 
print of approximately the same depth, upon toning it will be found 
that as the time of development is increased the color of the toned 
print becomes progressively colder in shade. ‘The student is advised 
to repeat this experiment in order that he may see for himself the 
exact effect of variations in exposure and development on the color of 
the toned image. From the series of prints so obtained it should not 
be difficult to select one which has the color desired. It will then 
serve as a guide for the development of future prints on the same 
paper which are to be toned. 

Owing, however, to differences in the temperature of various batches 
of the developing solution, the oxidation of the developing solution 
when in use, the reduction in its activity with use owing to the re- 
straining action of liberated bromides and to differences in the de- 
veloping speeds of various batches of the same paper, it is advisable to 
adopt the factorial system rather than to adhere to a straight timing 
method. The print to be used for sulphur toning should be developed 
to a rather high factor—the exact factor to be used depending some- 
what on the character of the emulsion and to a certain extent on the 
color desired. Once, however, the factor has been found which with 
a given emulsion produces a print which upon toning results in the de- 
sired color, duplicate prints of the same quality can be made at any 
future time. Where the total duration of development is so short as 
to make the factorial method inconvenient, as is the case with most of 
the developing papers of the gaslight type which are designed prima- 
rily for the use of the amateur, the adoption of a fixed time of de- 
velopment is perhaps the more satisfactory solution. In this case, 
however, care should be taken not to overwork the developing solution 


‘eo |) ho eo 2 
+ ee Let ce: 


460 PHOTOGRAPHY 


or uniform colors will not be obtained. Indeed for best results it is 
preferable to use fresh solution for each print, taking for this purpose 
only sufficient solution to cover the print. 

When development is conducted for a definite time, rather than by 
the factorial method, the amount of soluble bromide present in the 
developing solution has a very great influence on the resulting color. 
Therefore when for any reason time development is used the amount 
of soluble bromides added to the developing solution should be care- 
fully standardized in order that it may be possible to obtain the same 
tone in the future. Where development is by factor the amount of 
soluble bromide in the developing solution has but slight influence and 
the exact amount is therefore relatively unimportant. It is well, how- 
ever, in most cases not to use very much more than is Bia to pre- 
vent fog. 

The nature of the emulsion has considerable influence upon the re- 
sulting color. Asa rule, the faster the emulsion the colder is the tone 
obtained with normal treatment while the slower grades tend to pro- 
duce warm tones. These differences, however, may be, and often are, 
overshadowed by the exposure and development of the print. Thus, 
while it may be said that emulsions vary in their tendency to produce 
warm or cold tones, practically speaking, any desired tone within the 
range of the toning process used may be obtained when one has a. 
black print of the requisite character. 

In the case of sulphur toning by the indirect processes perfect fixa- 
tion is a matter of vital importance. The investigations of Lumiere 
and Seyewetz } have conclusively proved that the staining of the whites 
met with in sulphur toning by the indirect process is due solely to im- 
perfect fixation. They have shown that fixation is incomplete in a 
bath of hypo which contains more than 2 per cent of dissolved silver 
bromide and that prints fixed in such a bath will show a coloration in 
the whites on toning regardless of the time which they are left in the 
fixing bath. The authors therefore recommend the use of two fixing 
baths, one of which has been slightly used while the other is absolutely 
fresh, and the prints given ten to fifteen minutes’ fixation in each bath. 
It would be well as a matter of principle if workers would accustom 
themselves to the use of two fixing baths and discard the older of the 
two at regular intervals depending upon the number of prints fixed, 
its place being taken by the second bath which is in turn replaced by a 
fresh solution. 


1 Brit. J. Phot., 1923, 70, 732. 


TONING OF DEVELOPED SILVER IMAGES 461 


Sulphur toning processes such as hypo-alum and liver of sulphur 
(when used in a solution sufficiently strong) which contain a relatively 
large amount of hypo in their composition are without staining action 
on the whites of the print provided the fixation has been reasonably 
complete. 

Since these processes contain hypo as a constituent part there is no 
necessity for thorough washing in order to secure complete elimination 
of hypo. Thorough washing, however, is a matter of importance when 
the indirect processes are used since these are quite sensitive to its 
presence. 

The Hypo-Alum Process.—Of the several methods of direct sul- 
phur toning, the hypo-alum process is perhaps the most popular and 
is extensively used in American studios, practically to the exclusion of 
all other methods of sulphur toning. It is withal an excellent method 
for securing such colors as are within its range which may be said to 
extend from a slightly purplish-brown through various shades of brown 
to warm-chestnut-brown. It is regular and reliable in action produc- 
ing agreeable tones and free from the tendency towards extreme 
warmth of tone which makes it well adapted for many emulsions which 
with other processes produce disagreeable yellowish tones. 

The following is a reliable formula for the hypo-alum toning bath: 


ae tlh: 400 gm. 
ee ee ey. va as cence ke pes chee 80 fl. oz 2000 Cc 
aN E I ie ag asc pn ce kde ss eeree es 3% oz. go gm. 


Stir the solution well when adding the alum, then raise to the boiling 
point and boil for three minutes. Allow the mixture to cool and add 
the following silver solution, known technically as a ripener, which 
prevents the bleaching of the prints in the hot toning bath: 


eR er sy cone sis ks cbse ga din wa ewes 20 er. I gm. 
De ES SRS 5) Se 1 il, oz, 30 cc. 
Ammonia (.880) sufficient to redissolve the pre- 

cipitate first formed. 


The solution should be stirred vigorously while the ammonia is being 
added. It is then added to the hypo-alum mixture and the following 
solution of potassium iodide made up and added to the bath which is 
now complete and ready for use: 


Potassium iodide.......... DROW neers eae aE 40 gr. 3 gm. 
Pup Reem Mr RI Nn Si dee sly cs aces bv dace bn ca bele i fy oz: 30 CC. 


a 


462 PHOTOGRAPH? 


The prints for toning, which should be slightly darker than required 
since there is a slight bleaching action in toning, should be fixed thor- 
oughly and rinsed in several changes of water, then immersed in the 
hypo-alum solution which should be heated to a temperature of about 
go° F. (32° C.). The prints are kept on the move while toning in 
order that there may be no danger of uneven toning from the over- 
lapping of the prints in the solution. At the same time the tempera- 
ture of the bath is gradually raised to 110 to 135° F. (43-57° C.). 
The temperature of the toning bath has a slight influence on the color 
of the toned print, the warmth of tone increasing with higher tem- 
perature. The temperature should therefore be regulated with respect 
to the degree of warmth desired in the finished print. The prints are 
allowed to remain in the hypo-alum mixture until there is no doubt 
that toning has proceeded as far as it will go. There is no danger of 
over toning as the action proceeds to completion and then stops. The 
time required for toning varies with the temperature of the bath and 
with the emulsion, varying from 15-30 minutes at temperatures from 
I10 to 130° F. (43-54° C.). 

When fully toned, the prints are removed and the surface swabbed 
with hot water by means of a tuft of absorbent cotton in order to re- 
move the precipitate of alum which forms and are then washed and 
dried in the usual way. The toning bath itself should not be thrown 
away but bottled up for future use as it improves with age. 

Several methods of accelerating the action of a hypo-alum bath have 
been advised. Thermit in the British Journal of Photography ? recom- 
mends that the prints after having been fixed in a plain hypo fixing 
bath be immersed in a 10 per cent solution of sulphuric acid for half 
a minute, then transferred to the regular toning bath where the action 
will proceed quite rapidly. W. E. A. Drinkwater in recommending a 


similar method adds to the sulphuric acid solution a small amount of 


hypo, as follows: 


Sulphuric acid. ..5 Vin se ae ee t fh.o2 6.5 cc 
Water cos xacc bs cece ee gheaece een ee 150 fl. oz. 1000 Cc. 
Hypo’ ca ..'s 6c 6 5 waibee 0 00F 4 Oz. 26.6 gm 


Prints transferred from this solution to the regular hypo-alum bath at 

a temperature of about 110° F. (43° C.) tone completely in a few 

seconds. . 
Zanoff’s Controlled Hypo-Alum Process.—With the exception of 


2 1922, 69, 120. 


TONING OF DEVELOPED SILVER IMAGES 463 


the slight control possible by varying the temperature of the bath, when 
the ordinary hypo-alum process is followed the tone of the finished 
print is determined once for all by the exposure and development of 
the print. Zanoff, however, has described a variation in the usual 
process by which there is greater control over the resulting tone in 
the operation of toning. The formule for the two toning solutions 
required are as follows: 


(ou) SI Si i a 20. OZ: 156.25 gm. 
RS ee cy eev vee eaes 2 OZ. 15.6 gm 
Bouline water (distilled). .o...05..... 128 oz 1000 = “tc 


Boil two minutes, allow to cool and then add: 


SRT DO OSOUATE.. 5. os te ete we eee 2.02. 15.6 gm. 
(OO SS re 60 gr. .o5 gm. 
esc es, ie ic x nis 8 $0 0'0'e'¥-9 8 I Oz. 7.8 gm. 
Wrermaertirhy- OTOMMGG. 0.6.65 ee we ew 180 gr 2.85 gm 
ea desc vgakeeis I Oz. 78 gm. 


Pour the bromide solution into the solution of silver nitrate and add 
precipitate and all to the cool hypo-alum bath. 


RR POT IG o dace y ec c eh ele be ees 15 gr: .24 gm. 
UL ae ee re Ate Ae haem 2 OF: 150. -Cc; 


a hak pene sekansiy ee 16 oz. 125 em. 
SS" aS RS a a 4.02. 31.25 gm. 
ey Sid ue hb aid eine os 128 oz. 1000 CC. 


Boil five minutes, then cool and add the following solution which has 
been prepared separately : 


To eosin dc kv aes ew want 30 er. .48 gm. 
Beoremetn BrOMHNGE. 6 occ ct eee 30 er. .48 om. 
WE 8 5 sc 5 eee ps 8 be we ¥ 02; 7 Oo CG: 


The prints are first immersed in the first bath for six or seven 
minutes, according to the warmth of tone required, then rinsed and 
immersed in the second bath until toning is complete. The longer 
prints are left in the first bath the colder is the final tone. Accord- 
ingly by regulating the time of immersion in this bath the tones may 
be regulated to meet the desires of the operator, so that the action of 
the bath is completely under control. 

The first solution is used at a lukewarm temperature; the second at 
the normal temperature of the ordinary hypo-alum bath.* 


8 Abel’s Photographic Weekly, 1921, p. 224; Brit. J. Phot., 1921, 68, 680. 


464 PHOTOGRAPHY 


Sulphur Toning with Acid Hypo.—When an acid is added to a 
solution of hypo, the latter is at once decomposed, one of the products 
being finely divided sulphur. A number of toning processes based 
upon the decomposition of hypo by an acid have been brought forward 
although none have come into extensive use. Processes of this nature 
were brought forward by Lumiére and Seyewetz and H. Soar in 1914, 
by G. S. Hoell in 1915 and by the Eastman Research Laboratory in 
1922.4 In the method advised by the latter the prints are first im- 
mersed in a 5 per cent solution of sulphuric acid for ten minutes, then, 
after a brief rinse, in a 20 per cent solution of hypo saturated with 
borax. 

Toning with the Polysulphides.—A cheap and simple method of 
sulphur toning and one which produces acceptable results on many 
emulsions consists in the use of a polysulphide, usually in the form 
of the inexpensive “liver of sulphur,” a mixture of potassium poly- 
sulphide and potassium sulphate which usually contains certain im- 
purities in the form of potassium carbonate and potassium thiosul- 
phate. 3 

The following formula is recommended : 


“Liver of sulphur’ :...0.. 4.3 eee YA oz 12.5 gm 
Hypo... 66 ot ioe ee 1 Oz. 25 gm. 
Water}: ci 06 bien eee elena Gane ee 20 OZ 1000 cc 


As the temperature of the toning bath must be about 80 to go° F. 
(27-32° C.) and since the “liver of sulphur ”’ itself has a softening 
action on gelatine it is well to harden the prints before toning in a solu- 
tion of chrome alum, unless an acid fixing bath containing alum has 
been used in which case the degree of hardening will probably be suffi- 
cient. As the toning solution itself contains hypo, only a brief wash- 
ing is required before toning. Toning requires from ten to fifteen 
minutes at the above temperature during which time but little outward 
change in the color of the print will be observed. As the subsequent 
washing proceeds, however, and the yellow discoloration disappears 
the final color of the print becomes apparent. Blue spots arise from 
the presence of iron, due either to the use of trays in which iron is 
exposed or to the use of impure “liver of sulphur.’ The remedy is, 
in either case, obvious. 

E. Underberg advises the use of ammonium polysulphide which he 


4 Brit. J. Phot. Almanac, 1914, p. 66; Phot. Era, March 1915, p. 127; Brit. J. 
Phot., 1922, 69, 73. 


- 


TONING OF DEVELOPED SILVER IMAGES 465 


prepares as follows:° A stock solution is prepared by dissolving pure 
sulphur in commercial ammonium sulphide until the point of satura- 
tion is reached and then decanting the clear solution which keeps well. 
For toning, from ten to fifteen drops (.3-.6 cc.) of this solution are 
added to ten ounces of water (284 cc.) which is heated to a tempera- 
ture of 85 to 95° F. (30-35° C.). Toning proceeds rapidly and is 
complete within 5-10 minutes. Underberg considers this method one 
of the best because of its simplicity and regularity, the warm tones 
produced, and because the action is progressive, so that the action can 
be stopped at any time, in this way securing an intermediate tone due to 
the admixture of the original black with the toned image. 

Lumiere and Seyewetz, who investigated the use of “liver of sul- 
phur ”’ as a toning agent very thoroughly,® are of the opinion that the 
action of liver of sulphur on the developed image is comparable to 
colloidal sulphur and that the course of the reaction is as follows: 


S+H,O=—H,S + 0, 
Ag, + O= Ag,O, 
Ag.O + H.S —Ag,S + H,O. 


As the actual toning agent in the above case is the polysulphide con- 
tained in the liver of sulphur it is plain that this might be used alone. 
The solution of potassium polysulphide may be prepared by the method 
described by Bullock.’ 

Dissolve one hundred grams of potassium hydroxide in water and 
make up the solution to a total of 1000 cc. (in English measures I 
ounce to a total volume of to fluid ounces). Saturate one half this 
solution with hydrogen sulphide and mix with the remainder. To this 
solution, which is substantially one of potassium sulphide, add 120 
grams (1.2 ounces) of pure sulphur in powder, heat to the boiling 
point and boil for five minutes stirring rapidly all the time. The 
- potassium pentasulphide solution thus formed is then allowed to cool, 
filtered and kept in a rubber-stoppered bottle tightly closed. 

For use take 950 parts of water, 50 parts of the potassium penta- 
sulphide stock solution and 2.5 parts of a 20 per cent solution of am- 
monium sulphide. This bath as prepared will remain clear for about 
an hour after which the sulphur may begin to separate out. 

5 Brit. J. Phot., 1924, 71, 50. 

© bru, J, Phot., 1023, 70, 733. 

7 Brit. J. Phot., 1921, 68, 451. 


466 PHOTOGRAPHY. 


The time of toning is from 15-25 minutes at ordinary temperatures 
but the action may be greatly accelerated by the addition of either 
potassium sulphocyanide or potassium selenocyanide. With the ad- 
dition of 2 per cent of sulphocyanide the rate of toning is approxi- 
mately doubled. Increasing the amount of sulphocyanide increases 
the rapidity of toning but leads to a more purplish tone, which, how- 
ever, may, in some cases, be an advantage. 

Single Solution Sulphide Toning Processes.—A process of sulphur 
toning in which a solution of sodium sulphide containing an oxidizing 
agent together with some body to take up the caustic soda formed was 
described by Milton B. Punnett * in 1908, and similar processes were 
described by Dr. F. Kropf * in 1910, by E. Blake-Smith ?° in 1911, and 
in a leading article in the British Journal of Photography the previous 
year, while Dr. W. Triepel ™ patented under British Patent No. 24,- 
378 of 1910 a process of like character. 

The most successful of such methods, however, is that introduced 
by Mr. W. B. Shaw in 1923 in which the réle of the oxidizing agent 
is filled by nitro aromatic derivatives, such as, for example, nitro-ben- 
zene, sodium meta-nitro-benzene sulphonate and sodium 4-nitro- 
toluene 2-sulphonate. The toning solution is compounded from two 
stock solutions as follows: ™ | 


Saturated solution of barium sulphide: ....0..2. 150 .)see I5 parts 
10 per cent solution sodium meta-nitro-benzene sulphonate............. I part 


Both solutions keep well if stored in tightly corked bottles. A fun- 
goid growth, however, will form in the nitro-benzene solution with 
time. This growth may be prevented by adding to the solution a small 
piece of thymol. 

Prints immersed in this solution tone to a good sepia or brown 
within three to five minutes at a temperature of 60° F. (16° C.). The 
tones vary considerably with the emulsion; the greatest variation be- 
ing observed with papers of the gaslight type. With bromide papers 
there is comparatively little variation in tone with different emulsions. 
The tones resemble very closely those obtained by other methods of 
sulphur toning and since the method is one which works rapidly, is 

8 Brit. J. Phot. Almanac, 1908, p. 653; Brit. J. Phot., 1910, 57, 860. 

® Phot. Rund., 1910, 21, 245; Brit. J. Phot., 1910, 57, 836. 

10 Brit. J. Phot., 1911, 58, 140. 


11 1910, 57, 835. 
12 Brit. J. Phot., 1923, 70, 759. 


TONING OF DEVELOPED SILVER IMAGES 467 


simple in composition, produces agreeable tones and does not require 
the application of heat it will undoubtedly develop into one of the 
most popular methods of sulphur toning. 

The Indirect Process of Sulphide Toning.—Of the many methods 
of producing sepia and warm-brown tones, the indirect process is one 
of the most reliable and is perhaps in wider use than any other 
method. The well-washed prints are first bleached in one of a num- 
ber of baths, the most prominent example of which consists of potas- 
sium ferricyanide and bromide, briefly washed and toned in a solu- 
tion of sodium, ammonium or barium sulphide. As the latter has a 
softening action on gelatine the use of an acid fixing and hardening 
bath is advisable particularly in summer or when high temperatures 
cannot be avoided. 

For bleaching the print any of the following mixtures may be used: 


A. Ferricyanide-bromide (B. J. Almanac Formula) 


PIRATE UCOTIIGE, 6.66 ec ve eve ete en eees 100 gr. il Sm. 
Prermeerery BOPeICVANICE, 60. kc ee ee dca es 300 gr. ace om, 
ey ce Re an ar ar aa 20 OZ. 1000 cc. 


B. Permanganate (T. H. Greenall) 


(a) Hydrochloric acid (31.8 per cent)....... 3 02. iSO: 7 ec. 
Dee PROS AIC 6 dine 6h. k is iss eue ees nies eee 20 Oz. 1000 CC. 
Coie ceeesicm permanganate... ... 1... sees 40 er. 4.5 gm. 
NR es nlc ek fee ite ce ees wees 20 OZ. T1000? =cC: 


For use take one part each of (a) and (0) to 6 of water. Both 
a and b keep well in tightly corked bottles. This bleacher has the 
advantage that any traces of hypo left in the print are destroyed. 
With the ferricyanide-bromide bleach the presence of hypo leads to 
reduction of the print since the two interact to form the well-known 
Farmer’s reducer. The stain which results from the use of perman- 
ganate is usually removed in the sulphide bath, however, should it re- 
main after sulphiding, the print is immersed in 


ee ea gt vig: si bneie se vain midgea dw ks Y Oz. 10 cc. 
ORE ee eye se ae isso 8 e's ea ge neler Od 50 oz. 1000 cc. 
SE OO ES ge SF i ne I 0z 20 gm 


until removed. 
The following phosphate-ferricyanide bleacher is recommended by 
Mr. T. H. Greenall as giving colder tones than the usual ferricyanide- 


468 PHOTOGRAPHY 


bromide mixture. It may therefore be of value where a particular 
emulsion tends to produce an undesirable warmth of tone.*® 


Sodigm phosphates. 6s). cos ass eee 200 gr. 100 gm. 
Potassium “ferricyanide s.r ‘40 gr. 20 gm. 
Water <iatecieek Ge ess se Re ee 4 OZ. 1000 cc. 


Numerous other methods of bleaching have been advised from time 
to time by various workers. It is doubtful, however, if there is any 
real advantage in their use as compared with the usual ferricyanide- 
bromide mixture for, as has been shown by Bullock in an admirable 
paper on the subject of sulphide toning by the indirect process, the 
composition of the bleacher has comparatively little effect on the re- 
sulting tone." 

When the bleaching action is complete and it is seen that no further 
action takes place, the prints are removed and washed either in run- 
ning water or in successive changes until the yellow discoloration is 
removed. The length of this washing has no great influence on the 
final tone if within reasonable limits, but too long a washing is to be 
avoided while it is equally important that washing be sufficient to 
entirely remove every trace of the stain from the ferrocyanide. 
When washed the prints are ready for sulphiding. 

However, if at this point the prints are immersed for about ten to 
fifteen seconds in a I per cent solution of sodium carbonate, then 
rinsed in water and sulphided, a much cooler tone is obtained. This 
mode of procedure, indicated first by Bullock,*® is especially ad- 
vantageous for some emulsions which tend to produce extremely 
warm tones. 

The conversion of the bleached image into a colloidal silver sulphide 
can be accomplished by a number of substances, the more important 
of which are sodium, ammonium and barium sulphide. On account 
of its lower cost, sodium sulphide is generally used. Ammonium sul- 
phide is claimed by some to give richer tones but the author has never 
been able to find much difference in this respect provided both sub- 
stances are pure and other conditions are alike in both cases. 

Sodium sulphide does not keep at all well in solution and for the 
best results it is advisable to make up a fresh solution at the time 
of use. This may conveniently be a I per cent solution. 

13 Brit. J. Phot., 1912, 59, OI. 


14 Brit. J. Phot., 1921, 68, 447. 
45 Brit. J. Phet., 1921, 68, 447. 


TONING OF DEVELOPED SILVER IMAGES 469 


Sa ee ne oe 44 er. 10 gm. 
EE er cals ds'x eielecaibcs sie a4 5 wisn metas «vs 10 Oz. 1000 cc. 


There is no advantage in the use of a solution stronger than I per 
cent and there is the danger of blisters to be considered with solutions 
much above this strength. Above a concentration of I per cent the 
tone is not appreciably affected by the strength of the sulphide bath ; 
below this point, however, the tone is yellowish and lacking in vigor, 
the results approaching the normal as the concentration of the sul- 
phide solution reaches one per cent. 

While in general the use of stock solutions of sodium sulphide can- 
not be strongly recommended (partially decomposed sulphide from 
such solutions being one of the most frequent sources of failure in in- 
direct toning), relatively strong solutions of sulphide, if carefully made 
with distilled or boiled water and kept in tightly corked bottles using 
rubber stoppers, will keep for two or three weeks. Weak solutions 
do not keep well and it is advisable to make a fairly strong solution 
which has to be diluted considerably with water immediately before 
use. 

The use of barium sulphide was advised by Namias in rgr1. It 
has the advantage of keeping better in solution than either the sodium 
or ammonium salt; of being partially free from the objectional odor 
of the latter and producing colder tones. Its effectiveness, however, 
varies considerably with different emulsions, some of which refuse 
to tone. The salt is only sparingly soluble in water and a saturated 
solution forms about the most convenient strength for general use. 
This is made as follows: 


BEV eC IOC a Gig ys lee ek cee wiee cee thuuvanes 1 Oz. 12.5 gm. 
OS Ea SE oe 0 a a 4O Oz. 1000 cc. 


The principal objection to the use of barium sulphide, aside from 
its unsatisfactory performance with some emulsions, is the precipita- 
tion of an insoluble barium compound on the face of the prints. This 
may usually be removed by rubbing briskly with a wad of wet ab- 
sorbent cotton and may be entirely prevented by the addition of a very 
small quantity of sodium sulphate to the stock solution of barium 
sulphide. | 

The use of the sulphydrate was advised by Douglas Carnegie ’® in 
1907, on account of its better keeping properties and freedom from 


16 Brit. J. Phot. Almanac, 1907, p. 676. 


470 PHOTOGRAPHY 


objectional odor. It has never been extensively used for this purpose, 
however. | 

Re-bleaching of Sulphide-Toned Prints.—Prints toned by the 
indirect process may be re-bleached and developed should the color 
of the toned print be unsatisfactory. For this purpose the following 
bleacher is recommended: 


A; Tydrochloric acid. 225.v'a. hs eee ee 10 per cent solution 
B.- Potassium:permanranates 2. .ssaeue won 5 per cent solution 
Por ase take: Abia si. 249 de i eee I Oz. 250 cc. 

Bis oa 5h salciond Seng te ee 30 min. 15.8 tc, 


After bleaching the stain from the manganese is removed by the use 
of oxalic acid or sodium bisulphite, as previously described, and the 
print can then be developed in any ordinary developer or toned to a 
darker sepia by re-sulphiding. 

Indirect Sulphide Toning with Intermediate Redevelopment.— 
Practically the only control which the worker has over the color 
of the toned image in the case of sulphide toning as previously de- 
scribed is in the production of the original print. There is a means, 
however, of rendering the process more responsive to the desires of 
the worker. This means consists in partial redevelopment of the 
bleached image by means of a weak developer immediately before sul- 
phiding. The intermediate redevelopment combined with sulphiding 
has the effect of producing an image consisting partly of metallic silver 
(the redeveloped image) and of silver sulphide; the relation between 
the two images determining the tone of the print. 

For this manner of working the prints are bleached and washed 
in the ordinary way, then immersed in the regular print developer 
which has been diluted with from 10-15 parts of water and contain- 
ing no bromide. The use of a weak developer causes the image to 
develop slowly and gradually so that its action can be easily followed. 
The extent to which this intermediate redevelopment is carried de- 
termines the final tone; the longer the period of development the 
stronger is the black silver image and the colder the tone of the final 
result. When dealing with batches of prints the use of a short stop 
bath to arrest the action of the developer is necessary as its action 
grows more rapid towards the end. This stop bath may be the 
regular acetic acid bath as recommended on page 403 of the chapter on 
Printing on Developing Papers. 

After having been rinsed in the short stop bath, the prints are 


TONING OF DEVELOPED SILVER IMAGES 471 


washed for several minutes in running water or in several changes 
of water and then transferred to the sulphide bath in which toning 
is completed. ‘They are finally washed thoroughly in water and dried 
as usual. 

For successful working of this method careful attention to the 
following points is essential : 

1. The operations of bleaching and redevelopment must be con- 
ducted in artificial light not daylight. 

2. To obtain a uniform tone on all prints from the same negative 
the intermediate development must be alike for all. As the develop- 
ing solution is a weak one, care must be taken not to overwork it. 
In fact, it is best to take a small quantity of fresh solution for each 
print. 

When one has become accustomed to the appearance of the print 
in the developer and the effect of the extent of redevelopment on 
the final tone, this method of working becomes quite easy and certain 
while its latitude has obvious advantages."” 

Mercury-Sulphide Toning (Bennett’s Method).—In another process 
of sulphide toning introduced by Mr. H. W. Bennett the tone of the 
print is controlled by the addition of mercuric chloride to the bleach- 
ing solution. The addition of. mercuric chloride to the usual ferri- 
cyanide-bromide bleaching solution results in an image which con- 
sists partly of a compound of mercury and partly of silver sulphide; 
the tone depending upon the relative proportions of the two com- 
pounds. 

The following formule are the revised formulas given by Mr. Ben- 
nett in 1921.** 


Saper (ueanity pierricyanide. ....0 0.668 ee. es Lge Oz. 100 gm. 
GSR COMING: ois cares Cea ge deo oes 14 oz. 150. gm. 
ee eI gcd cia kms Gale war wile < acne: OO: 1000 cc. 

Pe TOU ECM OTIC oo sop ees ms ttn na ees pes 60 gr. 12.5 gm. 
PUM OUUGC) (ys ips ee ee ea ek we ae Ocul gr. 12.5 gm. 
UMPC RIOPINAE. ig kei ve kee eee eeees [pha oy 2 500 cc. 

oy CGS rh a ar ae hy Weer 100 gm. 
Sy Ver einai 9 a do Ae Oy Oz: 1000, > CC. 


17 A method of combined development and sulphiding in a single solution has 
been described by Mr. T. H. Greenall (Phot., 1912, p. 91; Brit. J. Phot. Almanac, 
1913, p. 659), but has no advantages and some disadvantages over the method de- 
scribed. 

18 Brit. J. Phot., 1921, 68, 25. 


472 PHOTOGRAPHY 


The use of mercuric chloride has the effect of producing a certain 
amount of intensification which may be compensated for by reducing 
the amount of exposure or the time of development of the original 
print so as to produce lighter prints which will, when toned, be of the 
proper strength. 

The following table shows the composition of the lice wee solu- — 
tion for various tones and the relative exposures required for the 
original prints designed for toning: 


Color A B Water Rel. Exposure 
Normal sepia'>422 beng ee ae 40 parts | — parts 480 parts | 10 seconds 
Gool septa “- 7 Aone. gone pee 40 20. ons 480 Ougset 
Colder.sepia- os is eee RON ean 30 ae 480°" anaes 
Browneblackiin 20.6.5 ones eae a0 rite Pre heaeey 480 “ ose pes 
Engraving black ie a a erthee rey SNe 480.0% G. g2577 


After having been bleached in a bleacher compounded as directed 
for the tone desired, the prints are washed briefly and passed through 
three successive baths of 1 per cent hydrochloric acid after which 
they are again washed and finally sulphided as usual. 

The engraving black tone, it may be mentioned, is not the cold 
neutral black of the ordinary developed print but is a purer, richer 
black, resembling very closely the tone of an etching or engraving. 
It has been the experience of the writer that the best results are se- 
cured on bromide papers and that these vary considerably in adapta- 
bility to the process. 

Toning with Copper. —_Dr. J. M. Eder ?® in conjunction with V. 
Toth claims to have been the first to show that silver prints could be 
toned to a reddish tint by treatment with cupric ferricyanide. Later 
Namias showed that copper salts mixed with potassium ferricyanide 
deposited red ammoniacal oxide of copper on silver prints.*° Eder 
on returning to the subject in 1900 advised the use of cupric sulphate, 
ammonium carbonate and potassium ferricyanide.2* The same year 
Mr. W. B. Ferguson, as the result of a long series of experiments, ad- 
vised the use of cupric sulphate potassium ferricyanide and a neutral 
citrate such as potassium citrate which he found far superior to am- 
monia or ammonium carbonate which had been advised by Namias and 
Leer: 


19 Eder, Phot. Korr., 1876. 

20 Namias, Phot. Korr., 1894, 327. 
21 Phot. Korr., 1900, 36, 537. 

22 Phot. J., 1900, 25, 133. 


TONING OF DEVELOPED SILVER IMAGES 473 


Two solutions are required as follows: 


(British Journal Formula) 


mma RRA Og os cpg wads sae 'eicin'y boa 60 gr. 7 gm. 
Feeneemin citrate (neutral). .5....cs6..ac0c--s 240 er. 28 gm. 
Pee hig ns ik disk Cups wane eve ea 20 oz. IO0O cc. 
eee MEU FERTICVANICE, «6.6. c ces vee ee sees 50 er. 6 gm. 
Pere UIE CULT AIES ee kk ede es cava vaeas 240 er. 28 gm. 
Pete sd hoe ee oka Ue dae evi oa Sa dawld’ 20 Oz. 1000 Cc. 


For use take equal parts. Should the prints appear purplish in the 
highlights increase the amount of potassium citrate in either A or B. 

The range of colors obtainable with copper toning extend from 
warm-black through varying shades of brown on to red chalk, the ac- 
tion being progressive so that the various tones follow one another in 
a definite order as the action proceeds. There is no intensification as 
with uranium and the results are quite permanent. 

According to Namias the tone is much improved if the prints after 
toning are immersed in the following bath for fifteen minutes: 


RNR ys Gh ca ub vA vee nes des ev ce 154 gr. 20 gm. 
PA MRIMPII SIT OMLOTICC Gs csi. oslo vc bala as dee va eases akc of) 50 gm. 
DE NOES CMY OS ok pia konpa So Mlei's needs aoe « 77 min. IO cc. 
eM IA vince cadre ace waded 16 Oz. 1000 Cc. 


This bath may be used repeatedly and keeps quite well. As some silver 
chloride is formed, refixing is necessary, but since an acid bath would 
have a reducing action on the image a plain hypo bath with a concen- 
tration of about 5 per cent should be used. 

Namias has lately recommended another method of copper toning.?° 
In this the prints are first bleached in 


MMe PAE leet ive dials wou ce ee eck odd ey wale 614 gr. 80 gm. 
PUeGte eT CUTALCY 6 cic en vcd cance nc we wane 8I er. 10.5 gm. 
EE OS a a ee er 16 Oz. 1000 CC. 
Pee eT OVA N IC seks oe ce a aeeine ene + oe 73 oF. 9.5 gm. 


After bleaching the prints are washed well and redeveloped in a metol- 
hydrochinon developer containing 0.2 per cent potassium bromide. If 
development is carried out in daylight the silver ferrocyanide is reduced 
while the copper ferrocyanide is unaffected. As the process has some 
intensifying action the prints should be somewhat lighter than actually 
required. 

Toning with Uranium.—The range of tones obtainable by toning 


221i. Prog. Fot., 1915, p, 347; Bull. Soc. franc. Phot., 1922, 64, 26. 


474 PHOTOGRAEIES 


with uranium extends from warm-black, through various shades of 
brown to plum colors and various shades of red, terminating in a 
bright brick-red. The toning action is progressive, the various colors 
appearing in a definite order as the action is allowed to proceed. Ow- 
ing to its intensifying action, uranium toning is not a process for dark, 
fully developed prints and prints which are to be toned with uranium 
should be made somewhat lighter than is required of the finished 
result. ) if 

As regards the permanency of prints toned with uranium there is 
some question. While in many cases the results are reasonably perma- 
nent, except for a slight metallic luster which forms around the edges, 
in other cases the toned image does not appear at all stable. While 
much is no doubt due to improper manipulation during and after ton- 
ing, when all is said and done, prints toned with uranium cannot be 
said to be very reliable. 

There are numerous methods of toning with uranium. We give 
Sedlaczek’s method, which, if not the best, is one of the best, having 
behind it the experience of a man who has devoted much time to the 
subject. : 

As a result of theoretical reasoning and considerable research, Dr. 
Sedlaczek recommends the following formula: *4 


Uranyl nitrate...) cv pascasica ae an ee 38 er. 5 gm. 
Potassium citrate. s..5.-.9 <4 5 oe te ee 38 er. 5 gm. 
Potassium ferricyanide. ...\..4 7.50 se 15 gr. 2 gm. 
Ammonia ‘alums. 5.5 Ooi ee ns ee ee 77 gr. 10 gm. 
Pure hydrochloric acid..¢ 5a, eee 2 min DLS: ec. 


Water. (oi) bisa Beaded cee ae 16 oz. 1000. cc. 


It is a matter of considerable importance that the print be thoroughly 
washed as the uranium bath is decomposed by hypo, producing stains 
which cannot be removed. 

A print immersed in the above solution shows virtually no change 
in the first half-minute, after which a slight brownish coloration be- 
comes apparent which finally deepens into a reddish-brown. The 
colors produced with a bath of the above composition are far superior 
to those produced by the older methods, being darker and richer owing 
to the presence of some of the black silver image. 

The removal of the yellowish coloration after toning is greatly 
facilitated by the use of the following bath: 


24 Phot. Ind., 1924, p. 234; Amer. Phot., 1925, p. 8. 


TONING OF DEVELOPED SILVER IMAGES 475 


STP uni S252 a) SO a ae a 38 er. 5 gm. 
Sodium sulphate (not sulphite)..........0. ceca 192 gr. 25 gm. 
Paes Orr has 4 4 5 See ge hie Re EY Bogert eg Ra 16 oz. 1000 cc. 


_ Three or four such baths may be required to remove the yellowish 
coloration entirely. The print should then be washed in running 
water free from alkalis for a minute or so, then fixed for five minutes 
in a 0.5 per cent solution of hypo followed by washing in running 
water made acid by the addition of 0.1 per cent of glacial acetic acid. 

The impermanence of uranium-toned prints, of which so much 1s 
heard, is due, according to Dr. Sedlaczek, to the omission of after fix- 
ing, or to the omission of treatment with hydrogen sulphide. Uranium- 
toned prints thus treated may be considered reasonably permanent. 
For this latter treatment either of the following sulphiding baths may 
be employed : 


MP ICG yh Silden bi cea cd ese we nes IQ gr. 2.5 gm. 

bo 21 Sn 8 min. i tec 

DC ee ki a keh ests Se eat souse 16 Gz... {| 1000. te. 
or 

DR RRC ies gee ea's. is ad od ayes he na 8s wT Se: 10 gm. 

PRC ACH hy es kee ck keer seeks wee 23 min. 2 CG: 

Bye ety Ta gro oe a 16 oz. 1000 cc. 


Combining the fixing bath with cobalt produces colder tones tending 
to violet as the amount of cobalt is increased. For this purpose the 
following formula may be suggested as a beginning: 


PN eS herd ba ek ewe be Geen 35 gr. 5 gm. 
Peer tt ee a dh gals ass Cc wale ba ieae wit 8 er. I gm. 
CE ky ees ovens saves. PrN ie Bi veer 8 gr. I gm. 
Rees Mea ee i ed, ya) Bleck ek wacee eee bela 16 oz. 1000 cc. 


Iron Toning Processes.—The use of toning processes employing 
salts of iron is rather limited, being confined principally to blue ton- 
ing, although by combination with uranium or by sulphiding green 
tones may be obtained. | 

For a blue toner the following formula is recommended : 


(B. J. Almanac Formula) 


Ferric ammonium citrate (10 per cent solution)...... 2 G2: 10" ce, 
Potassium ferricyanide (10 per cent solution)........ 2 Oz. IO cc, 
Preece acin, 110 per cent solution) ......04 04s es acco ws 20 Oz. 100 CC. 


476 PHOTOGRAPHY 


The well-washed prints are immersed in this solution until the re- 
quired tone is reached, then washed in running water until the whites 
are clear. 

Green tones may be obtained by toning with iron and sulphiding. 
The green tone is due to the combination of the blue image (produced 
by toning with iron) and the yellow silver sulphide produced by sul- 
phiding. Three stock solutions are required: = 


A. Potassium ferricyanide. 20), poe, eee eee 5 gm fee tge. ¢ 
Ammoniaie A. ts on. 58 ee eee 5 drops 
Water to amakedic, spescde ieee 4 rede ae 100 cc 3% oz 

B. Ferric. ammonium, citrate..5s. 12. see 2 gm. 3327 
Hydrochloric: acidi(conc.) o.).2 ste eee 5 cc. 80 min. 
Water to make. .o. 24 2s oh sine ee 100 cc. 3% oz. 

C.. Sodtum sulphuie cease te - cae a got eee I gm. is )gr, 
Watet sgs.nd. Wak ie ce oe ee 100 cc. 3% oz. 
Hydrochloric atid: ((cont.). 9424.0 pa eee 5 ce. 80 min. 


The well-washed print is placed in A until completely bleached, then 
washed free from stain, placed in B for four or five minutes, rinsed 
two or three times in plain cold water and finally transferred to C for 
5 minutes. A short washing in running water completes the process. 
‘ The purity of the whites of the print depends upon the washing fol- 
lowing the bleacher and it is therefore necessary that this operation be 
thorough and complete. The pale blue of the highlights which is so 
observable on the wet print generally disappears on drying. 

Toning with Vanadium.—Apparently the first description of va- 
nadium as a toner was made by Prof. R. Namias in 1go1.7° The 
method adopted by him was to immerse the print in a solution of a 
ferricyanide and then into a solution containing a vanadium salt. The 
normal color of the silver image toned with vanadium is yellow and in 
1903 Namias introduced the following formula for obtaining green 
tones—the green tone being due to the presence of a blue ferriferro- 
cyanide image and the yellow vanadium ferrocyanide : 7° 


Ferric chlortdé: |; 2 i nck es sa 4.8 gm. 23. EP, 
Vanadium chloride: olcs os eon eee ee 4 gm. 17.5 gr. 
Ammonium chloride...) ee ee 10 gm. 43.8 gr. 
Hydrochloric: dad (cones)... ina ee eee 10° ec 48 min, 
Waters. cc0. oo) ieee a ee 1000 CC. 10 OZ, .' 


The objection to this, as well as all early methods of vanadium toning, 


25 Eder’s Jahrbuch, 1901, p. 171. 
26 Eder’s Jahrbuch, 1903, p. 158. 


a 


TONING OF DEVELOPED SILVER IMAGES 477 


is that the solutions used contain a chloride and hydrochloric acid so 
that there must be some silver chloride formed and this has the effect 
of reducing the transparency of the image and hence its brilliancy. 
Mr. E. J. Wall?’ has worked out and described a method in which 
this objection is overcome by the use of either the oxalate or sulphate 
of vanadium. | 

Either of these salts can be made quite easily from ammonium meta- 
vanadate which is a comparatively inexpensive salt. To make the 
oxalate place 100 grams (3 0z. 230 grains) of ammonium metavanadate 
in a beaker or evaporating dish and add 460 grams pure oxalic acid. 
To this add 500 cc. (17 oz. 287 minims) distilled water stirring con- 
stantly all the while and then heat the mixture. As the temperature 
rises it forms at first a thick paste which becomes more fluid as the 
temperature rises while the color changes from white to orange-red 
and finally to a dirty gray-green. More water may be added and 
heating continued until a perfect solution is obtained. The color will 
then change to a brilliant blue and the total bulk of the solution can be 
made up to 1477 cc. (52 oz.) when we have a.20 per cent solution of 
vanadium oxalate containing a slight excess of oxalic acid. 

The actual toning solution is compounded as follows: 


Vanadium oxalate solution (20 per cent)... 50 CC. Y% fl. oz. 
(xalic acia (Sattirated solution)........... 50 cc. Y fl. oz. 
Ammonia alum (saturated solution)....... 50 cc. Y% fl. oz 
Ferric Oxalate solation. . 0.2.6.0... core. eee quant, suff. 
CO oer ny ee 50 cc. 1% fl. oz. 
Potassium ferricyanide (10 per cent solu- 

CMRI AEG eI arg odo ls is “ba dno 6 v9 ms 10 cc: 48 min. 
WA Sa ip les ie Ge Gl ge ge ea 1000 cc 10 oz 


To prepare this solution add the oxalic acid to the vanadium and add 
half the water, then add the alum solution and then the ferric oxalate. 
The only means of determining the exact quantity of this is by trial. 
The more used the bluer the toned result. The ferricyanide should be 
mixed with the glycerine and the other half of the water, then added 
to the remainder of the solution. This will result in a bright, clear, 
green solution Which will not precipitate while toning. As it is sensi- 
tive to light, however, it is best to use it by artificial light. 

The alum is added for the purpose of keeping the highlights clear 
and the acid helps to keep the solution while in use. The glycerine is 
not absolutely necessary and may be omitted if desired but the bath is 
then more likely to produce a deposit. 


27 Phot. J. Amer., 1921, 57, 96; B. J. Almanac, 1922, p. 305. 
32 


it fi 2 eee 


478 PHOTOGRAPHY 


Toning requires from ten to fifteen minutes, after which the prints 
are to be immersed in a Io per cent solution of sodium sulphate for 
five minutes, washed briefly and dried. Fixing is unnecessary. 

Minor Toning Processes.—There are a number of minor processes 
of chemical toning of limited application owing to the unsatisfactory 
character of the result or to the difficulty of securing consistent results. 
Recent investigations of some of these processes have shown that they 
are capable of considerable improvement and it appears quite likely 
that in the future some of them, at least, may be more widely employed 
than at the present time. This-is particularly true of toning processes 
involving the use of stannous and cobaltic compounds. In both of 
these fields considerable development has taken place in recent years, 
largely as a result of the work of Formstecher and of Druce in the 
case of the processes with stannous salts and of P. Strauss with cobalt 
processes. The reader is referred to the published papers of these 
workers (a list of which will be found in the bibliography following 
this chapter) for further information on these processes. © 

There has likewise been a renewal of interest in processes of sele- 
nium toning and a number of patents have been taken out, and several 
papers published on toning processes involving the use of selenium. 
It has not as yet, however, come into general use, except in a limited 
way with certain products for which it has proved especially suitable. 
The same is true of several processes employing hydrosulphite as 
worked out by A. Steigmann and for colloidal silver processes de- 


scribed by Lumiére and Seyewetz, Rawling, Formstecher, Shelberg 


and others. | 

In a work on the toning of photographic images these minor proc- 
esses would of necessity assume considerable importance. In a gen- 
eral work, such as this, lack of space prevents a lengthy treatment of 
such processes as are not in general use. 


Books 


Braxe-SmitH—Toning Bromides and Lantern Slides, 1904. 
FRAPRIE—How to Make Prints in Colors. 

Meses—Der Bromsilber und Gaslicht Papier Druck, 1913. e 
SEDLACZEK—Die Tonungsverfahren von Entwicklungspapieren, 1906. 
STENGER—Die Kopierverfahren, 1926. : 


CHAPTER XXI 


PRINTING WITH SALTS OF IRON AND PLATINUM 


THE PLATINOTYPE PROCESS 


Introduction.—Platinum is one of the most stable of metals. It 
is affected very little by the strongest alkalis and not at all by sul- 
phuric, hydrochloric or nitric acids nor any substance found in the 
atmosphere. It follows, therefore, that prints, the image of which 
consists of pure metallic platinum, are as stable as the paper on which 
they are made. Not only are platinum prints permanent but they 
also have a certain intrinsic quality that is not possessed by any other 
process. Perhaps no printing process can reproduce all the original 
gradations of a good negative so faithfully, while many shades of 
sepia, warm and engraving black of unsurpassed purity may be easily 
obtained. Platinotype is also one of the simplest processes to manipu- 
late. 

The sensitiveness of certain salts of platinum to light was observed 
by Sir John Herschel in 1832 and by Hunt in 1844, but the develop- 
ment of the process is due to W. Willis, an Englishman, who took 
out a patent for the first practical method in 1873; a second followed 
in 1878, and a third in 1880. The work of two Austrian investigators, 
Pizzighelli and Hubl, also deserves mention more particularly for their 
work on the direct printing-out method. Their little book + is a com- 
plete treatise on the subject and despite its age is still one of the best 
textbooks on the process. 

The Theory of the Process.—Although platinum salts in the plati- 
nous state are sensitive to light, particularly in the presence of organic 
matter, the present platinotype process is an indirect one depending 
upon the reduction of a ferric salt to the ferrous state upon exposure 
to light and on the fact that this latter when dissolved in a solution of 
potassium oxalate is capable of reducing a platinum salt to the metallic 
state. ; . 

Paper is coated with potassium chloroplatinite (K,PtCl,) and ° 


1 Platinolype, translated by Abney—published by Harrisons, London, at 2 
shillings. (Now out of print.) 
479 


480 PHOTOGRAPILY 


ferric oxalate (Fe,(C,O,),) and dried. On exposure to light the 
ferric salt is reduced to the ferrous state in proportion to the amount 
of light action and upon immersion in potassium oxalate solution the 
ferrous salt is dissolved and the platinum salt with which it is in con- 
tact is reduced to the metallic state. Berkeley’s formula, which is 
generally accepted, is as follows: 


6Fe(C,0,) + 3PtKeCh, a 2Fee(C20x4)3 -f Fe.Cle “bo 6KCI <b 2P 


Ferrous Potassium Ferric Ferric Potassium 
oxalate chloroplatinite —> oxalate + chloride + (Platinum 
chloride) 


The remaining salts are dissolved in baths of dilute hydrochloric acid, 
leaving an image consisting of metallic platinum. 

Commercial Papers and their Treatment.—Platinum paper is sup- 
plied in a wide variety of surfaces and in black and sepia. The 


paper is extremely sensitive to moisture and therefore it is sent out: 


in sealed metal cans which contain a small quantity of-a moisture- 
absorbing salt so as to keep the paper dry and in good condition. 
The can should not be opened before the paper is to be used and then 
only in a dry place. When opened the can should not be allowed 
to lay around open in the workroom but the paper which is needed for 


immediate use should be removed and the remainder again placed in 


the can and the latter sealed. If it is necessary to remove a few sheets 
at irregular intervals, it is advisable to insert a freshly dried piece of 
calcium chloride each time in order to take up any moisture which 
the paper might have absorbed while the can was open. Too much 
care cannot be taken in keeping the paper dry, for if kept dry it wih 
remain in good condition almost indefinitely, while if allowed to ab- 
sorb moisture from the atmosphere, it will spoil very quickly and 
yield flat and lifeless prints. 

For best results the negative should have a little more contrast than 
is necessary for soft gaslight paper although the contrast can, to a 
certain extent, be controlled in development. For a full scale print 
the extreme shadows of the negative should be free from fog and 
the highlights plucky yet not blocked up. A little experience will 
quickly show the proper kind of negative. Thin, under exposed nega- 
tives are not suitable as the process reproduces all the faults as well 
as all the beauties of a negative. For some reason, a moderately thin 
negative is better than a dense one and care should, therefore, be 
taken when making the negative not to overtime. 


SALTS OF IRON AND PLATINUM 481 


Exposure.—The paper is very sensitive to light and should be 
handled only in artificial or exceedingly subdued daylight as it is 
from three to four times as fast as the silver print-out papers which 
we have been considering. Very bright daylight should not be allowed 
to reach the paper nor should it be exposed to the direct rays of a 
strong artificial light for any length of time. 

The negative, which should be thoroughly dry, is placed in the 
frame with the emulsion side up and the paper placed with its sensi- 
tive side in contact with the negative in the same manner as has been 
described in connection with other print-out papers. One essential 
_ difference, however, consists in the use of a sheet of waxed paper or 
vulcanized rubber over the paper to prevent the access of mois- 
ture from the atmosphere while exposing. This is especially im- 
portant on damp or dull days when the exposure is prolonged but is 
always to be advised. The progress of printing is examined in 
exactly the same way as with gelatine P-O-P, greater caution, how- 
ever, being taken not to expose the paper to strong light while the 
examination is being made. It is rather difficult to describe the ap- 
pearance of the paper when the exposure has been sufficient but the 
precise moment at which the exposure should terminate is easily 
gained with a little experience. For the beginner the best guide that 
the author can give is the following: When the image is fully seen in 
brownish gray against the yellow surface of the paper and full detail 
can be seen in the shadows the exposure 1s sufficient. In the majority 
of cases, however, especially with papers that are old or those that 
are home-made, a test should be made. Over exposure will, of course, 
give a dark print while under exposure will give a light print lacking 
detail in the highlights. 

Development.—The chemicals necessary for developing may be 
obtained from the American agents Willis and Clements of Philadel- 
phia in %4 Ib. packages or the following formula may be used for all 
grades of black papers: 


SE G00 5 oz. 33-5 gm. 
ee 15 Oz. 1000 Cc. 


Hot water is required in making up the solution but the developer 
should be allowed to cool before using. It keeps indefinitely and may 
be used over and over, sufficient fresh solution being added to it from 
time to time in order to keep up the required volume. 


482 PHOTOGRAPHY 


Since the image appears almost immediately, the paper must be 
immersed in the solution in such a manner that it is evenly and quickly 
covered in one sweep. Any air bubbles which appear should be 
carefully removed with a soft brush or the tip of a finger. With cor- 
rect exposure, there is no fear of over development and after a full 
minute’s immersion the print may be removed and immersed in a 
clearing bath composed of 


Waterirecs cc. cas Sieh eieeee eh ea oe 60 oz. 1000 cc. 
Hydrochloric acid ‘C.P.es. 2. ae I Oz. 16.6 cc. 


After five minutes’ immersion in this bath the print should be trans- 
ferred to a second bath of similar composition for five minutes and 
then to a third for another five minutes after which it is washed for 
fifteen to twenty minutes in running water and dried. Prints dry 
better when hung by the corners on a line than when placed between 
blotters. 

Variations in Contrast.—For softer prints one of the following 
modifications must be made: 


a. Heat the developer and print slightly less. On no account, how- 
ever, should the temperature of the developer exceed 180° F. 

b. The addition of a small amount of hydrochloric acid, say 1 drop 
C.P. to each ounce of developer. 

. Old paper gives less contrast than fresh. | 

. Some authorities recommend that printing be conducted under 
signal green glass but in the writer’s experience the increase in 
contrast secured in this manner is almost insignificant. 


SS 


For greater contrast: 


a. The developer may be diluted and greater time allowed for its 
action. It should not, however, be diluted further than one 
part of the normal solution already given to 4 parts of water. 

b. The addition of small quantities of potassium bichromate to the 
developer. The amount for a given negative can be deter- 
mined only by experience, but only a small quantity is needed 

- and the addition of three or four drops Io per cent solution 
to each 16 of developer has a considerable effect. In no case 
should there be more than I grain to Io ounces of developer 
used. - 

c. Diluting the developer with an equal part of glycerine and clear- 
ing in a strong acid bath. 


When using the last named method the image requires to be some- 
what darker than usual. Development is conducted in the usual man- 


fae OF TRON AND PLATINUM 483 


ner but owing to the restraint exercised by the glycerine the action 
is slow and the shadows develop more rapidly than the highlights. As 
soon as the desired depth is reached, the print is removed from the 
developer and immersed in a strong acid bath to arrest further de- 
velopment. The bath for this purpose should be composed of 


SMa tg hee at TOG oe a wey acdsee 30 Oz. 1000 cc. 
Pee ACI GE ys scifi ks wis v heo ee Sede ences I OZ. 24.4 OC; 


Owing to the difficulties of adjusting exposure and avoiding streaks 
in development, the use of potassium bichromate is preferable for 
the purpose of securing increased contrast. However, many workers 
employ the glycerine method in order to secure the peculiar velvet 
effect which it gives because the image is held upon the surface in- 
stead of sinking within the pores of the paper. 

Variations in Color.—A special “sepia”’ paper is supplied by the 
Platinotype Company, but sepias and various shades of warm-black 
may be secured on the black papers by the addition of mercury to the 
developer. | 

In general the “sepia” paper is handled in the same manner as the 
“black” but the following points require separate mention. The 
paper is rather more sensitive to light than the black papers and, there- 
fore, prints faster and requires to be protected from the light with 
greater care. The following developer is recommended: 


PCM AC OCVOIOUET . . 4... s oe ieee eke swede tees IO parts—20 oz. 
AeanCumeraneatubated SOliutiON....5.. 0.) 00 cokes ee yo I part — 2 oz. 


or the special sepia developing salts sold by the Platinotype Company. 
For the best results, the developer should be used at a temperature of 
150° to 160° F.; but very good results, particularly with certain nega- 
tives, may be secured in a cool developer. 

Trays which are used for the development and clearing of sepia 
prints should be set aside for that purpose only and not used for black 
_ papers. Neither should the two papers be cleared in the same solu- 
_ tion nor should they be washed together. 

Very fine sepia tones may be secured on black paper by the addition 
of mercury. The use of mercury alone will degrade the highlights so 
glycerine must be added to retard its action. The following developer 
is advised by F. J. Mortimer: 


Pile tee At Dlacke GEVELOPEL. oui 3% sacca nis cde vurlee ewe’ I part—1I0 oz. 
RE RT Rae ha ats Shiga us gia eipmey 9% oun I part—I0 oz. 
B. to per cent solution of mercuric chloride in alcohol. 


484 PHOTOGRAPHY 


For use, A and B are mixed according to the tone desired—the larger 
the proportion of B the warmer the color. 
The following proportions are suggested : 


A 40 parts, B I part (a) 
A 30 parts, B 1 part (0) 
A 20 parts, B I part (c) 
A 20 parts, B 2 parts (d) 
A 20 parts, B 3 parts (e) 
A 20 parts, B 4 parts (f) 


(a) gives a warm-black; (b) brown-black; and (f) a warm-sepia ; this 

last is the maximum amount of B which it is permissible to use. For 

a given depth of printing (a), (b), and (c) give darker prints while 

(d), (e), and (f) give lighter prints, so it is necessary that an allow- 

ance be made in printing in order to secure prints of the desired depth. 
Another formula due to C. F. Inston is as follows: 


A, Potassium oxalate. 4... +5 sone ee 2 02; 142.8 gm. 
Water ani cexk ante nae . o's Ue on de 14 02. 1000 CC. 
B. Potassium ‘crtrate.: 3.0. ss ee 2 150 er. 21.5 gm. 
Citric -acid® st oe Se eee 240 gr. 34.3 gm. 
Mercuric chloride: 930.6 O34 (Ree go gr. 12.8 gm. 
Water snc UR a ee 14 Oz. 1000 cc. 


For use take equal parts of A and B and use at a temperature of about 
100° F. 

Mercury-toned prints should be cleared in a bath of about one third 
to one fourth the usual strength, say: 


Water oc coin JO, ee Cee a ee 200 Oz. 1000 cc. 


Care should be taken not to overwork this weak acid- ‘path or the prints 
will not be permanent. 

Silver-Platinum Papers.—Owing to the very high price of platinum 
in 1913, the Platinotype Company introduced a silver-platinum paper 
under the trade name Satista. The prints on this paper are excellent 


and practically indistinguishable from true platinotypes. They are — 


Juminous and full of atmosphere and the shadows rich and transparent. 
Moreover, the prints are reasonably permanent and the manipulation 
of the paper is very simple. The paper is very sensitive to moisture 
and must be kept in airtight tubes like platinotype. Exposure is con- 
ducted in the same manner as platinotype but as the paper is faster, 


SALTS OF IRON AND PLATINUM 485 


only about one fourth of the time is required in printing. The de- 
veloping solution consists of oxalic acid and potassium oxalate, with 
the addition of a small amount of ammonium chloride in the case of 
weak negatives to increase the contrast. After development, prints 
are cleared in a solution of binoxalate of potassium, washed for eight 
minutes in running water, fixed in a bath of sodium thiosulphate 
(“hypo”) and finally washed for thirty minutes in running water to 
eliminate all traces of the latter salt. The cost of the paper is about 
one third of platinotype and full supplies may be obtained from the 
agents, Willis and Clements, Philadelphia, Pa. 

Formulas for the preparation of similar papers have been published 
by Thomson.?. The following is the sensitizer advised : 


ee RI 0g ae ae 20 gr. 41.6 gm. 
Iron and ammonium citrate (green)........ SOM Tice? 41.6 gm. 
ee te en a a 20 gr. 41.6 gm. 
RAEN MHI cc cs leks ere ee dae 10 min. 20.8 cc. 
Potassium bichromate solution......... from 3-I0 min. 5.5-20.8 cc 
Oy TS COP 8 Ss Sa eS a IO gr. 20.8 gm 


Mix in above order and allow to stand for twenty-four hours. 

The paper may be sensitized either by floating or by brush and is 
dried in a moderately warm room. When dry, it is ready for exposure 
which is conducted in the same manner as platiriotype. 

The stock developing solution consists of : 


PMI RARE ee. iano ee dais Bebe OS 1.02; 1000 ~—- CC. 
ROMER ae ese educa we bdecsaeaeeds 4O er. 85 gm. 
Oe Sy Say Coie at HL a eae 10 gr. 21.25 gm. 
WIR AIG AEG ay cc ee nn vce vce wees Se eis IO gr. 21.25 gm. 


Filter and use clear solution, diluting for use with seven parts of 
water. ‘To secure pure black tones, the ferric oxalate must be ab- 
solutely fresh. If the image lacks strength, use a strong developer. 
Prints blacken immediately and after development are remoyed to a 
bath of hypo, 10 grains in six ounces of water (3.5 gm. to 1000 cc.), 
for ten minutes. Contrast may be regulated by the proportion of 
potassium bichromate (5 per cent solution), using from one to ten 
drops to each ounce of sensitizer (2-20 cc. to each 1000 cc.) accord- 
ing to the degree of contrast desired. 

The platinum solution named in the above formula is as follows: 


2 American Photography, 1915, Nov., p. 632. | 


486 PHOTOGRAPHY 


Potassium chloroplatinite; 6.0. Sate ee 15 gr. 15.6 gm. 
Phosphoric 4aGid pumiond cy fos Stes eee a aa 2 de: 150 ce: 
Distilled ‘waters sich Bes au.5 aieses Suivi: Bad ee I oz. 500 cc. 


When dissolved add water to make a total of two ounces (1000 cc.). 


The Kallitype Process.—Kallitype was the name given by W. W. 
Nicol to a ferric printing process in which ferric salts are reduced by 
exposure to light to the ferrous state and in this condition are able to 
convert a silver salt into the metallic state. The process is, therefore, 
similar to platinum excepting in the use of silver in place of platinum. 

Suitable paper is sized in a solution of arrowroot: 


Bermuda arrowroot... 2. s:cs.+ 5 sees ete go gr. 18.7 gm. 
Water’... (shea bes Se eee ee 10 Oz. 1000 CC. 


Using a little of the water make a thin cream of the arrowroot. Then 
heat the remainder of the water to the boiling point and add to the 
arrowroot mixture. As the solution does not keep it must be made 


up fresh for each batch of paper sized. The sizing solution is best 


applied with a Blanchard brush. 
When dry the paper is sensitized with: 


Ferric oxalate...c.diveccdsbunx Ss eames fee ae 75 gr. 15 gm. 
Oxalic acid. ou. 6d iis ache ele ou 5 er. I gm. 
Silver’ nitrate: Poco P se, ue Peaits oe ae ee 30 gr. 6 gm. 
Distilled iwater, cao one ee ae a Vo ake a eee I Oz. 100 cc. 


The ferric oxalate must be dissolved with the oxalic acid in warm 
water, then filtered and the silver nitrate added. 

The operations of sensitizing, drying, and exposing are as with 
platinotype. 


Both black and sepia tones may be had, depending on the developer 


used. For black tones the following is recommended : 


Borax > is Fics sike eae's ek ee ee eee I oz. 100 gm. 
Rochelle salt (oT po ae : Y% Oz. 75 gm. 
Distilled .watér. a: <suk, tages oe eee 10“ Oz. 1000 cc. — 
Potassium bichromate (1 per cent solution) 

according to brilliancy desired......... « 6-10... dr 75-125 cc. 


For sepia tones: 


Rochelle salt (sodium-potassium-tartrate)..... 1% oz. 50 gm. 
Potassium bichromate (1 per cent solution)... 4-6 dr. 50-75 cc. 
Distilled “water ois. oss bese gee ee 10 Oz. 1000 cc. 


Development is complete within ten to fifteen minutes. The length- 
ened time of development will not make the print too dark provided 
exposure has been correct. 


SALTS OF IRON AND PLATINUM 487 


After development the prints are rinsed briefly in water and fixed in: 


er iets fy ics pe un bs bene caves I Oz. 50) «6gm. 

WVAteRrN... ots ous NE. shes yay pak ow ne s 20 0z. 1000 cc. 

OS Se eee nee 120 min. 1Zi5 cc. 
(2 dr. fl.) 


after which they are washed for about thirty minutes in running 
water.® 

Blue Printing.—Blue printing, now in wide use by engineers for 
making copies of plans, etc., from tracings, was invented by Sir John 
Herschel in 1840, and called cyanotype by him. It is a ferric process 
depending upon the conversion of ferric salt to the ferrous state and 
the precipitation of Prussian blue by ferricyanide of potassium. Blue 
print paper can be obtained commercially in 3 grades: fast, medium 
and slow in most cut sizes and also rolls of various lengths. It does 
not keep well so no more should be ordered at a time than can be used 
in two or three weeks. 

However, it is easily made and the following directions are given 
for those who wish to coat their own. Smooth, thin paper is coated 
with the following solutions: 


(B. J. Almanac formula) 


Pier ORRaSI) TETTICVONICE. «5.5. 6. cos ee hve ee sans 1100 gr. 250 gm. 
Ne eS ii iue Hie inie Gv A ea ass 10 Oz. 1000 cc. 

B, Ferric ammonium citrate......... RS ae Dyers 400 gr. + QO gm. 
I ee CNS erin Cig s.5 4) ois pine » vos ack eon = 10 Oz. 1000 cc. 


For use take equal parts. Both solutions keep well in the dark. The 
ammonio iron salt must be fresh in order to secure good results. The 
solution is applied with a brush, working first in one direction and then 
the other in order to secure an even coating. After drying, it is ex- 
posed under the tracing until the shadows are bronzed. Washing in 
running water for fifteen minutes terminates the process. ‘The use of 
a ten per cent solution of potassium bichromate increases contrast and 
enables sufficient contrast to be obtained from weak tracings. As the 
bichromate solution bleaches the image the paper must be considerably 
over printed to secure sufficiently dark prints. 

By a modification of the above positive prints having blue lines on a 
white background can be obtained from ordinary tracings in which the 

83 For a comprehensive treatment of this and other variations of Kallitype, 


see Photominiature No. 185 by James Thomson. See also: American Photog- 
raphy, 1918, Nov., p. 642. 


488 PHOTOGRAPHY 


lines are in black ink on a transparent white background. Three solu- 
tions must be made up. 


TW atten. a abs signe cise. 9 5 dhcp nt stay ea ee 20 0z. ~—‘ 1000 cc. 
Gunisarabie. gan vee. 0.5). co o-0 hen cee ee 4 Oz. 200 gm. 
2. Water inte oid es Ds nc a 20 0z. —- 1000 cc. 
Ammonto-citrate of iron). ...2)4..saee eee IO Oz. 500 gm. 
So. Water as ce Cad iis «ole ia Suc eesti eG ae 20 Oz. 1000 cc. 
Ferric chloride. .......... + ab obedient seg cee Saga IO Oz. 500 gm. 


The above solutions will keep for a month or six weeks. For use 
they are mixed as follows: : 


Solution Nos Yo eos. os eajuis ca dis eee ab ble cack cele tn 30 parts 
Solution No, 2.......25¢.5.0++5-+ sp + oes or 8 parts. 
Solution No. 3.....c0.s0eccee chews e bse Seen 5 parts 


This is almost clear at first but gradually grows thicker and should be 
used soon after mixing. It is applied with a brush and it is to be 
noted that most papers require to be sized beforehand, while most 
papers do not require any preliminary sizing for the regular blue print 
process. : 

After exposure, the print is developed with a brush filled with 


Potassium: ferrocyanidé:. ... i... ts: 4.99 56s eee 200 gr. 104 gm. 
Water. iso c diacaallb ove bebe oa te eee ee ee 4 Oz. 1000 Cc. 


When every detail has appeared and the print is dark blue, rapidly 
rinse in water and place in a bath of commercial hydrochloric acid 
(one part to ten parts water) after which it is washed in running 
water and dried. Positive papers for making copies of drawings, etc., 
are sold under a variety of names and give not only blue but also 
black and brown lines on white backgrounds. 


GENERAL REFERENCE WORKS. 
PLATINOTYPE 


Pizzighelli and Hubl.—Platinotype—English translation by Iselin and Edited 
by Abney, Published by Harrison and Sons, London. 

Horsely Hinton—Platinotype Printing. 

Abney and Clark—Platinotype. 

The Photominiature No. 7—Platinotype Processes. 

The Photominiature No. 40—Platinotype Modifications. 

The Photominiature No. 115—Platinum Printing. 

George E. Brown—Ferric and Heliographic Processes. 

Photominiature No. 10o—The Blue Print. 

Photominiature No. 47—The Kallitype Process. 

Photominiature No. 81—Ozobrome, Kallitype and Blue Prints. 

Photominiature No. 185—Kallitype and Allied Processes. 


el 


GHAPTER -XXIT 


PRINTING PROCESSES EMPLOYING BICHROMATED 
~ COLLOIDS 


Part I. Historical 


Sensitiveness of Chromic Compounds and Bichromated Colloids.— 
The first to observe the sensitiveness of chromium compounds to 
light appears to have been Mungo Ponton, an Englishman, who in 
1839 discovered that paper soaked in a bichromate and dried was 
sensitive to light. The following year Becquerel discovered that 
when the paper was sized with starch it became more sensitive and 
he decided that the sensitiveness of the chromium compound was due 
to the presence of organic substances used for sizing the paper. In 
1852 Fox-Talbot found that when bichromate is mixed with gelatine 
and exposed to light the gelatine is rendered insoluble. In 1855 
Alphonse Poitevin discovered that if-a colored substance be added to 
gelatine, sensitized with potassium bichromate, and exposed to light — 
under a negative, the unaffected parts might be washed away leaving 
an image formed by the colored substance held in the insoluble gela- 
tine formed as a result of the action of light. This was the founda- 
tion of the carbon and gum-bichromate processes. Poitevin also dis- 
covered that a bichromated gelatine film when exposed to light and 
allowed to swell in water would take a greasy ink on the exposed 
portions but not on the unexposed portions. From this he developed 
a process of photomechanical printing known as collotype and at a 
later date the processes of oil and bromoil, based on the same prin- 
ciple, were brought out by others. Poitevin may thus be termed the 
father of printing processes employing bichromated colloids. 

The Development of the Carbon and Gum-bichromate Process.— 
In 1858 John Pouncy of Dorchester, England, was granted a patent 
for a carbon process based upon the same principles as that of 
Poitevin. His method consisted in brushing over paper a mixture of 
bichromatized gelatine and carbon: the paper after drying being ex- 
posed under the negative and developed in water. His results were 
far from satisfactory, however, because the half tones were lacking. 

489 


490 PHOTOGRAPHY 


The same year the Abbe Laborde showed the reason for this saying: 
“In the sensitive film, however thin it may be, two distinct surfaces 
must be recognized, an outer and an inner which is in contact with 
the paper. The action of the light commences on the outer surface. 
In the washing, therefore, the half tones loose their hold on the paper 
and are washed away.” } 

The same year J. C. Burnett, Blair and Schouwaloff to overcome — 
this defect suggested the expedient of exposing from the back of the 
paper, but in 1860 Fargier in France showed that the best way was 
to coat the exposed film with collodion, then transfer it to glass and 
then wash away the soluble gelatine from the back. This method, 
however, was too complicated for general use. a 

In 1864 J. W. Swan patented carbon tissue, which is simply paper 
coated with gelatine and pigment, which, after sensitizing in bichro- 
mate, is exposed under the negative and transferred before develop- 
ment to another support. The tissue backing is then stripped off 
leaving the pigmented gelatine on the new support. Development is 


effected by washing away the soluble gelatine in water. This was 


the first really practical process of pigment printing in which the pig- 
ment is incorporated with the bichromated colloid before exposure. 

The gum-bichromate process, now so popular among pictorialists of 
~ a certain class, is nothing more than Pouncy’s carbon process which 
he described before a meeting of the Photographic Society of London 
in 1858. It was brought to the front about 1895, largely as the re- 
sult of the work of Robert Demachy, Ch. Puyo and other French 
pictorialists. Abroad, it passed out upon the advent of the oil and 
bromoil processes but in America it has held its ground and is still 
popular in many quarters. 

In 1873 Marion? found that a sheet of paper immersed in bichro- 
mate and exposed to light so as to produce a faint image will transfer 
its image to a sheet of pigmented carbon tissue when the two are 
placed in contact with one another. This is due to the fact that there 
is left in the image some chromate of chromium, the salt formed as 
a result of the action of light on the bichromate, and this diffuses into 
the pigmented tissue and renders it insoluble just as. if it had been 
exposed to light. Manly in 1899 introduced? a process of pigment 
printing based upon this principle under the name of ozotype. This 


1 Brit. J. Phot., 1873, p. 342. 
2 British Patent No. 10,026/1899. 


PROCESSES EMPLOYING BICHROMATE Cit ELOms- agi 


failed to catch on, however, and in 1905 the same worker introduced * 
his ogobrome process. In this a sheet of ordinary carbon tissue was 
soaked in a solution containing potassium bichromate, potassium fer- 
ricyanide and potassium bromide. It was then placed in contact with 
an ordinary bromide print which had been previously soaked in water 
to become limp. 

After being kept under pressure for several minutes the carbon 
tissue was stripped from the bromide print, squeezed to its final sup- 
port and developed as usual. The bromide print thus takes the place 
of the negative, so that an enlarged negative is not required when 
prints larger than the original negative are desired, nor is daylight 
necessary at any stage. Ozobrome for a while was quite popular but 
finally fell into disuse. It was revived, however, in an improved form 
by Howard Farmer about 1917 under the name carbro and in its im- 
proved form has become so popular that it is again bringing carbon 
printing back to the attention of amateurs and professionals and may 
eventually supersede carbon, except where critical definition is re- 
quired. } | 

The Development of the Oil and Bromoil and Powder Processes.— 
A second process was worked out and patented by Poitevin in which 
a bichromated gelatine film without pigment was exposed under a 
negative. This gelatine film upon exposure to light under the nega- 
tive became more or less insoluble in various portions according to 
the gradations of the negative. When immersed in water, the soluble 
gelatine absorbs water and becomes so charged with water that it 
_will repel a greasy ink, while the shadows, being insoluble, do not ab- 
sorb water and will accept the ink. Accordingly when a roller 
charged with greasy ink is passed over the paint, an image is formed 
in greasy ink which adheres to the shadows but not to the highlights 
of the print. This process was the forerunner of a number of photo- 
mechanical processes, which are beyond the scope of this work, and 
the oil, bromoil and powder processes. 

Poitevin’s patent of 1855 is not very explicit, but in 1858 Asser was 
granted a patent for a process based upon the same principle and in 
which very precise directions were given. 

Two years previously the Duc de Luynes through the Societe Fran- 
coise de Photographie had offered a prize of 10,000 francs to the per- 
son discovering a process by which absolutely permanent prints might 


8 British Patent 17,007/1905. 


492 PHOTOGRAPHY 


be produced. The President of the Societe, M@. Regnault, the fa- 
mous chemist, in announcing the offer called attention to the perma- 
nency of carbon and suggested that experiments be conducted with a 
view to obtaining prints in carbon. Two Frenchmen, Garnier and 
Salmon, starting from Poitevin’s patent of 1895 worked out a process 
in which the bichromated gelatine was exposed to light under the 
negative, then soaked in water and pure finely divided carbon dusted 
over it. The carbon adheres only to the unexposed parts and in this 
way an image is secured. This was the beginning of the so-called 
powder processes. : 

Rawlings’ process of oil printing (1904) is actually little more than 
a modification of the process covered by Poitevin’s patent of 1855. 
Rawlings advised the use of brushes rather than a roller for applying 
the ink, thus making possible the control of the various tones of the 
print by varying the amount of ink deposited. This feature served 
to attract various workers who wished to have a ready means of alter- 
ing the tone values of their prints and the process rapidly gained in 
popularity among pictorialists. 

In 1889 Howard Farmer found that when a gelatine film contain- 
ing finely divided silver, as in a negative or positive, is immersed in a 
bichromate, the gelatine in contact with the metallic silver is rendered 
insoluble exactly as though it had been exposed to light in these por- 
tions. Upon this property of a brichromated colloid is based the 
bromoil process The first suggestion of the rationale of this process 
is due to E. J. Wall* It was taken up and worked out practically 
by C. Welborne Piper.® } | 

The Chemistry of Pigment Printing with Bichromated Colloids.— 
It must be remembered that in all the processes based upon the action 
of light on a colloid containing a bichromate, the pigment which forms 
the image is unaltered and plays no part in the reaction. It is just as 
easy to produce an image in pure bichromatized gelatine as with one 
containing a pigment, only in the first case the image would hardly be 
visible unless stained up by the use of dyes. The reaction involved is 
therefore simply one between light and a bichromated colloid. The 
chemistry of this reaction, however, is by no means clear. It appears 
that “ gelatine aided by light reduces the bichromate to a lower state 
of oxidation and then enters into combination with a compound of 

4 Phot. News, 1907, 51, 200. 

5 Phot. News, Aug. 16, 1907. 


ee a 


PROCESSES EMPLOYING BICHROMATE COLLOIDS 493 


chromic oxide produced by the mutual decomposition of the bichro- 
mate and gelatine. Using potassium bichromate we can assume that 
this may be represented as follows: 


pee tee thr, 1 Cr,O, — CrQ,. 


Cr,O, — CrO,, chromate of chromium, is a brown insoluble salt 
which combines with colloids to render them insoluble. Not only 
light but other reducing substances will reduce bichromate to the chro- 
mate of chromium. Among these is metallic silver which forms the 
basis of the ozobrome and carbro processes of Manly and Farmer. 


Part II. The Carbon Process 


Introduction.—Ever since its introduction as a practical process, 
carbon has been recognized as one of the finest of printing mediums. 
While it does not allow the same degree of control as some of the 
later processes, as gum-bichromate and the oil pigment methods, there 
is a good deal of latitude in carbon printing and if multiple printing is 
employed alterations of values may be effected to a considerable ex- 
tent. In common with other processes, which depend upon the action 
of light upon chromic salts, the carbon process affords a wide variety 
of colors and surfaces. The Autotype Company, who are the prin- 
cipal makers of carbon materials, supply tissues for over thirty dif- 
ferent colors. But even more important is the fidelity with which 
carbon reproduces the delicate tones of a negative. In this respect 
it is approached by no other medium and a carbon print will repro- 
duce the fine tones of a good negative more truthfully than any other 
process extant. | 

There are two variations in carbon printing known as single and 
double transfer. In the first case the image is reversed from right 
to left, while in the latter instance the image is non-reversed. Carbon 
tissue consists of paper coated with gelatine and pigment. Before 
use, it has to be sensitized in a solution of potassium bichromate and 
is then dried in the dark. With the spirit sensitizer, manufactured by 
the Autotype Company, the drying is very rapid and sensitizing is an 
operation which requires very little time. When dry, the tissue is 
exposed to daylight under the negative, a photometer being used to 
regulate the exposure as the image is not visible. When exposure is 
complete the tissue is removed from the frame and allowed to soak 


until limp in cold water. In the meantime, a sheet of single transfer 
33 


494 (PHOT OGRA 


paper, or the temporary support if double transfer is to be made, is 
allowed to become pliable in the water. As soon as limp, the two are 
brought together and pressed into contact. After remaining under 
pressure a short while, the two are immersed in warm water and the 


tissue stripped off, leaving the gelatine and pigment adhering to the © 


transfer paper, or to the temporary support in case of double transfer. 
Gentle washing in the warm water follows and the unacted upon 
bichromated gelatine with its pigment soon washes away leaving the 
image in pure insoluble pigment. As soon as development is com- 
plete, the print is removed, placed in a bath of. alum to remove the 
bichromate stain and harden the gelatine and is finally dried. Double 
transfer is a little more complicated. After development on the tem- 
porary support, the image is hardened and allowed to dry. It is then 
again placed in water and brought in contact with a sheet of double 
transfer paper. The two are allowed to dry in contact and the paper 
may then be stripped from the temporary support carrying with 
it the image. The introduction of spirit sensitizers has rendered car- 
bon a comparatively simple and straightforward process and instruc- 
tions unfortunately make it appear more involved than it really is. 

Carbon Tissues.—The Autotype Company of England are the 
principal manufacturers in the world of materials for the carbon 
process and supply everything necessary for working the process. 
Over fifty different tissues in thirty colors are made by this company 
and there are a large number of different transfer papers to select 
from, giving practically all useful tones and surfaces. The following 
is a list of the more important tissues of the Company: 


Terra cotta Cold bistre Cool sepia 
Ivory black Warm black Portrait purple 
Warm sepia Engraving black © Portrait brown 
Standard brown Sepia White cameo 
Standard purple | Red chalk Talbot sepia 
Gray green Ruby brown Sea green 

Dark blue Platine black Brownish black 
Blue black Italian green Vandyke brown 


Much may be done to enhance the effectiveness of the print by 
judicious choice of a color appropriate to the subject. Thus, Dark 
Blue or Sea Green is suitable for pictures at sea or on large bodies of 
water. Suitable colors for landscapes are found in Engraving Black, 
Ivory Black, Italian Green, Vandyke Brown, Warm Black and Gray 


PROCESSES EMPLOYING BICHROMATE COLLOIDS 495 


Green. Portraits appear to advantage on Red Chalk, Sepia, Standard 
Brown, Warm Black and Brown Black. A decided advantage of the 
colors obtained by the carbon or any other pigment process, over those 
obtained by toning, lies in the fact that the tones are purer and may be 
duplicated with ease and certainty which is rarely, if ever, the case 
with toning processes. 

Double and Single Transfer.—Carbon prints from glass negatives 
are reversed from right to left unless double transfer is employed. In 
the majority of pictorial subjects this is not objectionable and single 
transfer is quite suitable. Non-reversed carbon prints may be made 
from films by printing from the back side and with little or no loss in 
detail or definition. If carbon printing is proposed at the beginning 
the negative may be reversed in any one of several ways. The plate 
may be placed in the plate holder with the glass side facing the lens; a 
reversing mirror may be used, or the film may be stripped from the 
negative and reversed. This latter is a rather risky method to employ 
but some appear to have good success with it. At any rate it is best 
to use the special stripping plates for the negative as there is then less 
danger of trouble in the operation of stripping and reversing. On the 
whole, double transfer is to be preferred to any of these methods where 
it is necessary that the picture appear in the same manner as seen by 
the eye; 1.e., non-reversed. 

Sensitizing the Tissue—The tissue is supplied in packages of a 
dozen sheets in nearly all standard sizes and also in bands 2% by 12 
feet. In commercial establishments, the tissue is usually bought by the 
band but it is more convenient for the beginner to buy the ready cut 

tissue. Since the tissue tends to curl, it should be kept under pressure 
"until used, and as the solubility becomes less with age until complete 
insolubility is reached, no more tissue should be purchased at one time 
than may be used up in a few months at the most. 

The sensitizing bath consists of pure potassium bichromate. Only 
the purest form of this chemical should be used. That sold for storage 
batteries, etc., is unsuited. The strength generally advised for average 
negatives is four per cent solution, although with weak negatives better 
results will be secured with tissue which has been sensitized in a bath 
of lower concentration, as 2 per cent or 3 per cent. The sensitiveness 
depends upon the strength of the sensitizing solution and also upon the 
tissue. Thus tissue sensitized in a I per cent bath of potassium bi- 
chromate requires about three to four times more exposure than that 
sensitized in a bath of 4 per cent, while a color such as Red Chalk re- 


496 PHOTOGRAPHY 


quires more time for exposure than turquoise blue, owing to its greater 
opacity to actinic light. However, generally speaking, the tissues all 
require about the same exposure. 

On the whole, the use*of the Autotype Company’s Spirit Sensitizer 
is to be advised for the amateur or occasional worker. For com- 
mercial work, where the proper facilities are available for drying the 
tissue after sensitizing, the plain bichromate bath is satisfactory but as 
these are generally not at the disposal of the amateur, it is recom- 
mended that he make use of the spirit sensitizer, which dries very 
quickly, requiring no elaborate drying cabinet, and permits the tissue 
to be used within an hour. 

In place of the spirit sensitizer of the Autotype Company the follow- 
ing may be used: 


Ammonium” bichromaté..... 03.5. : . +e 60 gm. 460 gr. 
Water toomakes. 3. ...05 6) eae pe 1000 cc. 16 oz. 


This is a stock solution one part of which should be diluted with an 
equal volume of alcohol. The diluted solution will not keep. The 
tissue should be immersed in the diluted sensitizer for five minutes, 
then removed and treated as described later. 

The operation of sensitizing with a spirit sensitizer is as follows: 
Pour an ounce or two of the sensitizer into a saucer or cup and dip 
the Blanchard brush, supplied with each bottle of sensitizer, in the same 
and then apply to the tissue which should be pinned down on a board 
with pushpins. The solution must be evenly distributed and rapidly 
as it dries quickly. First cover the tissue lengthways and then dip the 
brush in the solution again and go over the tissue a second time in the 
opposite direction, namely the short dimension. For a large print, a 
special brush should be made in order that the surface may be more 
rapidly covered with the sensitizing solution. When the first sheet of 


tissue is surface dry (several sheets may be coated in the meantime), it — 


should receive a second application in order to insure a uniform coat- 
ing. When finished throw away the remaining sensitizer and wash 
out the brush and keep for future use. The sensitizing bath should 
be kept in the dark when not in use. The tissue should be hung on a 
line in a dark room to dry, which will take from ten to twenty minutes. 
The use of an electric fan will hasten drying as will also moderate 
heat. Of the two, the first is the safer, as heat may render the tissue 


6 For methods of sensitizing using dyes, see Meisling, Brit. J. Phot., 1916, 63, 


Feb. 23; Dansk. fotografisk Tidskrift Nos. 9 and 10, 1916; Warburg, Phot. J., 


1917, 57, 160. 


‘qa 


Te Oe ee) ween. a ee eS ee 


PROCESSES EMPLOYING BICHROMATE COLLOIDS 497 


insoluble. The tissue should be thoroughly dry before it is placed in 
the printing frame. If it is intended to keep the tissue for any time, 
it should be placed in the storage tube supplied by the Autotype Com- 
pany. Sensitized carbon tissue is at its best, however, as soon as dry’ 
and can only be kept in good condition with safety for a week or so, 
even in the special storage box. 

The tissue may also be sensitized by immersion but then requires 
much longer to dry. It is, however, more sensitive than that sensitized 
by brushing and increases in sensitiveness with age. 

Exposure.—No special type of printing frame is required, but since 
there is no necessity for examining the print during exposure, the back 
need not be made in two pieces as usual. 

The tissue must be kept dry while exposing and for this purpose 
sheets of waxed paper or waterproof sheets of vulcanized rubber, as 
used for the same purpose with platinotype, may be employed. The 
springs of the frame need to be strong and for this reason many of 
the cheaper frames known as “ Amateur ”’ will be found unsatisfactory 
since the springs are weak and unable to hold the rather stiff pig- 
mented tissue in perfect contact with the negative. 

Printing is usually done in the shade as the direct rays of the sun 
may crack or cause the tissue to become insoluble. For commercial 
work, the Cooper-Hewitt mercury vapor lamp is a satisfactory light. 

Before printing, the negative should be provided with a “ safe edge.” 
This is a narrow opaque border on all four sides of the negative which 
insures a soluble margin to the picture by protecting the tissue from 
the action of the light. This “safe edge” may consist of a strip of 
Opaque paint on the negative or the black paper masks sold for the 
purpose of producing prints with white borders. 

As the image is invisible, an actinometer is used to gauge the time of 
exposure. Several forms are ‘obtainable. Three popular types, Bur- 
ton’s, Sawyer’s, and Johnson’s, are illustrated in Fig. 221. 

In Johnson’s actinometer a small roll of sensitive paper 1s contained 
within the cubical box. This is pulled forward and exposed to light 
beside the frame until the tint of the paper and the standard tint 
register and a new piece pulled into position. A thin negative may be 
sufficiently exposed in one tint, a medium one in two or three, while 
denser ones range higher. Such actinometers are known as intermit- 
tent and are not so convenient as the continuous type of which the 
Sawyer is an example. In this instrument there is a graduated scale 
of increasing opacities ranging from 1 tog. The paper is exposed be- 


498 PHOTOGRAPHY 


neath the graduated scale and each number darkens in succession to 
the standard tint so that there is no necessity for moving the paper 
during an exposure. The Burton instrument is similar but has several 
portrait negatives of increasing density. Sensitive paper is placed 


_ JOHNSON SAWYER - 


Fic. 221. Actinometers for Carbon Printing 


under the negative in the actinometer which appears to resemble in 
density the negative to be printed and the two exposed until the test 
paper appears sufficiently dark. : 

Carbon tissue sensitized on a 4 per cent bath of potassium bichromate 
is about three times as rapid as P-O-P. The beginner will find it 
necessary to make two or three tests and after development the proper 
exposure can be judged. The number of “tints” required may then 
be marked upon the negative so that the proper exposure at any future 
time can be readily determined with the actinometer. 

After exposure the print should be developed as soon as possible as 


ee SS ee ee eee ee SOO 


PROCESSES EMPLOYING BICHROMATE COLLOIDS 499 


if left to stand the action of light will continue even though the print 
be kept in complete darkness and over exposure will result. This is 
known as “ the continuing action of light” and was first observed by 


Johnson, and Abney later showed that the principle might be used to 


advantage in increasing the speed of printing in dull light. It is pos- 
sible to work out a system whereby one third or even less of the 
original exposure may be given and the print allowed to stand several 
hours before development. Unless absolutely uniform conditions can 
be maintained, this ‘method is: not to be advised however and the be- 
ginner will do well to develop immediately after exposure. 

Development.—The development of a carbon print is a compara- 
tively simple operation. No chemicals are needed, hot water and a 
large tray being the principal requirements. No dark room is required 
and the operation may be carried out in subdued daylight. Up to this 
stage there is no difference in double or single transfer but before de- 
velopment it becomes necessary to transfer the pigment to either its 
transfer paper or to a temporary support from which it will later be 
again transferred to its final support. Transfer is necessary because 
the insoluble pigment which has been formed by the action of light is 
upon the surface of the tissue while the insoluble pigment which must 
be washed away in order to reveal the image lies beneath the image. 
It is, therefore, necessary to transfer the pigment so that the soluble 
pigment will be on top where it can be washed away without affecting 
the pigment forming the image. 

We will first consider single transfer as it is the simplest and best 
for the beginner. After he is able to make single transfer prints with 
satisfaction, he may experiment with double transfer. 

A sheet of single transfer paper is placed in cold water at about 60° 
F. (16° C.) for several minutes until it is limp.’ The exposed tissue 
is then placed in the same water. The tissue will at first curl inward 
and then outward until it becomes practically flat. At this point it 
should be removed from the water and placed face down upon the 
transfer paper, which should have been previously placed upon a flat 
surface with the side coated with gelatine upwards. When the two 
are in contact, a squeegee is passed over the same from center to the 
margin with moderate pressure in order to eliminate air and moisture. 
After being squeegeed into contact the tissue and its support may be 

7 Very rough or thick transfer papers shou!d be allowed to soak for an hour 


before being squeezed into contact with exposed tissue. With thin and smooth 
papers ten to fifteen minutes will be sufficient. 


500 PHOTOGRAPEY 


placed under blotters and allowed to remain for fifteen to twenty 
minutes before development. 

To develop, immerse the print and its support in water at about 95 
to 100° F, (35-38° C.). . In a few seconds the pigment will begin to 
ooze out around the edges. When this begins, separate the corner of 
the tissue and its support by lifting it with the finger nail and pull off 
the paper that originally held the pigmented gelatine. This tissue may 
be discarded. Holding the print by one corner, gently splash warm 
water over the surface. The soluble -pigment will gradually wash 
away leaving the image. Care should be taken not to touch the print 
with the hands or any hard substance as the gelatine is very soft and 
easily injured at this stage. If the print is under exposed, the pigment 
will wash away very easily while if exposure is excessive the pigment 
dissolves with difficulty and warmer water must be used. Highlights 
may be lightened or detail in dark shadows may be brought out at this 
stage by squirting water from a blow tube against the print, or hot 
water may be poured on the desired portion. 

When development is complete, place the prints in clean cold water 
for a minute and then transfer to a five per cent solution of alum to 
remove the bichromate stain and harden the gelatine. Porcelain steel 
enamelled or hard rubber trays may be used for the alum solution but 
tin or zinc vessels, such as may be used for development, are to be 
avoided. The time required in this bath varies but sufficient time 
should be given to make sure that all of the bichromate stain has been 
removed as any trace which is left will be more noticeable when dry 
than while wet. 

After clearing and hardening in the alum bath, the print is removed 
and well rinsed in water and then hung up to dry. Carbon prints 
should not be forced in drying by heat as there is a danger of the 
gelatine cracking. An electric fan, however, may be used to hasten 
the process. : 

Double Transfer.—So far we have considered only single transfer 
which is quite suitable in all cases in which reversal of the image is not 
objectionable. The operations prior to development are the same in 
both single and double transfer. Before development, instead of being 
attached to the transfer paper, the exposed carbon tissue is fixed to a 
temporary support. ‘This temporary support may be opal glass or the 
specially coated paper supplied by the Autotype Company. This latter 
product is made in two grades: Thick No. 112 for general use, giving 
either medium gloss or matt; and Thin No. 112 which is advised for 
thick and rough surfaced transfer papers. 


PROCESSES EMPLOYING BICHROMATE COLLOIDS 501 


Before use, the support must be waxed so that the gelatine image 
may be stripped from the temporary support without danger when 
transferring to the final support. The waxing solution consists of one 
part of beeswax and three parts of resin dissolved in turpentine and 
may be purchased especially prepared. Several drops of this waxing 
solution are poured on the temporary support and gently rubbed over 
the surface using a pad of flannelette. The waxing is a simple opera- 
tion but care must be taken that the support is evenly and thoroughly 
waxed. Unequal distribution gives rise to a difference in surface 
texture on the finished print as some sections will have more gloss than 
others. If a part of the support has not been covered at all the 
gelatine may adhere and the print will then be spoiled. An hour or so 
must be allowed after waxing before the supports are used in order to 
allow the turpentine to evaporate. It is an excellent plan to wax the 
supports several hours, or on the day before they are to be used. 

There is less latitude in development with double than with single 
transfer and the exposure should be as nearly correct as possible in 
order that very hot water may not be necessary for development. 
There is a tendency for the image to blister while on the temporary 
support which is due to a softening of the wax and the use of hot 
water, of course, will cause the wax to soften more than cold. 

The temporary support is placed in water along with the exposed 
tissue and allowed to remain until it becomes comparatively flat. The 
exposed tissue is then squeegeed to the waxed side of the temporary 
support and allowed to remain under pressure for several minutes 
after which it is developed in exactly the same way as single transfer. 
After development, the temporary support with its adhering image is 
placed in the alum bath to discharge the bichromate stain and harden 
the gelatine after which it is rinsed and allowed to dry. Care should 
be taken not to injure the delicate surface during any of these opera- 
tions. Dry the image in a cool place—not in the sun nor by any kind 
of heat. 

When dry, the operation of transferring the image to the final sup- 
port may be proceeded with. For this purpose double transfer paper 
is supplied in a wide variety of tones and surfaces. The sheet of 
double transfer paper should be larger than the temporary support, say 
7xQfor5x7print. It is placed in cold water and allowed to soak 
for an hour in order to swell the gelatine coating so that the image will 
adhere. After soaking for an hour remove and place for a minute or 
so in water at about 90° F. until the surface feels slimy to the touch 


502 PHOTOGRAPHY 


after which it is again returned to the cold water where it may remain 
until required. The dry print on the temporary support is now placed 
in cold water until flat and limp and is then taken out and placed face 
up upon a smooth flat surface as a sheet of plate glass. The sheet of 
softened double transfer paper is then placed on top of it and held in 
place by one hand while the two are squeegeed into perfect contact with 
a flat squeegee. The pressure must be sufficient to force out the water 
but not so great as to affect the gelatine. A few trials will serve to 
show the proper amount of pressure to apply. The temporary support 
and double transfer paper are then hung up on a line to dry. When 
thoroughly dry, insert the point of a knife blade under one corner and 
pull the two apart, when it will be found that the image leaves the 
temporary support and adheres to the double transfer paper. If the 
plate has not been properly waxed, the image may adhere to the 
temporary support in places and the print ruined. This may also 
happen if the gelatine coating of the double transfer paper has not been 
sufficiently softened before use. 

Transferring to Rough Papers.—It is strongly advised that the be- 
ginner stick to smooth surfaces until he is perfectly sure of himself. 
However, the carbon image can be transferred to very rough surfaces 
as Whatman’s hot pressed drawing paper, but greater care and famili- 
arity with the process is required and for this reason the beginner will 
do well to stick to smooth and moderately rough surfaces for quite a 
while. The temporary support to use is No. 112 thin. The image 
cn its temporary support is placed in water for several minutes and 
allowed to become limp. It is then removed and immersed in the 
gelatine solution which is prepared as follows: 

Soak one ounce of Nelson’s gelatine No: 1 in 20 ounces of water 
for fifteen minutes. Then heat the solution to about 115 to 125° F. 
until the gelatine is melted. To an ounce of water add 5 grains of 
chrome alum and when dissolved, add to the solution of gelatine, stir- 
ring well all the while. The gelatine solution should be strained 
through muslin before use. 

A piece of the transfer paper, which has been soaking in water for 
an hour or more, is then removed and placed face up on a smooth, flat 
surface. The print on its temporary support is removed from the 
gelatine solution and placed face down upon the transfer paper and 
the two squeegeed firmly into perfect contact. Clean the margins of 
the transfer paper, which should be an inch or so larger than the 
temporary support, and hang the prints up to dry. When thoroughly 


r- 


ae ee Te ne 


PROCESSES EMPLOYING BICHROMATE COLLOIDS - 503 


dry, they may be stripped off as usual. Any slight smoothing of the 
rough surface, due to being in contact with the smooth waxed tempo- 
rary support, may be removed by soaking the print in water for half 
to three quarters of an hour and redrying. 

The Carbro Process.—The carbro process has a number of note- 
worthy advantages over the older method of carbon printing and will 
no doubt serve to increase the popularity of pigment printing. Un- 
like carbon no daylight is required at any stage so that the difficulties 
of drying the tissue and exposing to daylight are avoided. This 
greatly simplifies the production of pigment prints and by rendering 
the worker independent of daylight enables him to use his evenings in 
printing. There is in addition the advantage that an enlarged nega- 
tive is not required when prints are desired larger than the original 
negative, since from a good bromide enlargement any number of 
carbros in reason may be made without loss of quality. In these days 
of small cameras and dependence upon projection printing the possi- 
bility of using a bromide enlargement instead of an expensive en- 
larged negative is a matter of some moment and this point weighs 
heavily in favor of the carbro process. As a carbro print is identical 
with a carbon print made from the negative by the usual method, it 
is evident that we have in carbro printing the same range as regards 
color and texture that we have in carbon, and, since the finished print 
is the same in both cases, the same features of artistic excellence for 
which the carbon process is noteworthy. Added to this there is the 
possibility of multiple printing, which is simpler in carbro than in any 
other process. There is one objection to the carbro process which 
makes it technically inferior to direct carbon prints made from the 
negative, and that is the loss of critical sharpness. This,'as pointed 
out by Namias,* is due to the fact that the image is detached from the 
pigmented layer of gelatine and there is a local spreading of the action 
owing to the lateral diffusion of the insolubilizing agent within the 
pigmented gelatine. However, for all but the most critical scientific 
work where extreme sharpness of minute detail must be preserved, 
the slight softening of the outlines is unobjectionable and in the case 
of pictorial work may be actually an advantage. 

It is therefore not too much to say that carbro represents a notable 
advance in carbon printing. It is deserving of the attention of every 
serious amateur and professional and will, no doubt, do much to re- 


8 Brit. J. Phot., 1913, 60, 141. 


=p ee 7 


504 PHOTOGRAPHY 


vive the waning interest in the carbon process which remains, as it 
has always been, one of the finest of positive printing processes. 

The Bromide Print.—Since in carbro printing the bromide print 
acts as the negative, its preparation should be with the same care and 
attention which would be bestowed on a negative. As regards the 
make of paper, nearly all commercial brands of bromide appear to 
be suitable. The times of immersion in the various baths, however, 
differ with various brands of papers, but this is a matter of minor 
importance since the time of immersion once determined for a given 
brand remains constant, within small variations, for succeeding 
batches of the same paper: On the whole the platino matt or semi- 
matt surfaces are perhaps the best grades to employ as they are easier 
to work with and give a larger number of prints. Gloss papers, how- 
ever, may be used as well as the rough grades. In the case of very 
rough surfaces there is some difficulty and until the worker is thor- 
oughly familiar with the process he will do well to avoid such papers. 

The bromide print should receive full but not over exposure and 
must be fully developed. The factorial method of development al- 
ready advised for bromide prints is strongly recommended for the 
development of the original bromide to be used for carbro printing. 
A slight burying of the shadows in the bromide print is not an ob- 
jection since, owing to the superior gradation of carbon in the shad- 
ows, such gradations, although lost in the bromide, will be visible in 
the carbro print. 

While in general bromide papers yield the finest results, gaslight 
papers may be used when necessary. In this case the gaslight print 
should first be bleached in the usual ferricyanide-bromide bleach as 
used for sulphide toning and redeveloped in amidol or metol-hydro- 
chinon. It will then produce carbros equal in every respect to those 
made from bromide prints. The use of gaslight paper is at times 
desirable when owing to the character of the negative it may be im- 
possible to get a print of the required contrast on bromide paper. 

As in carbon, it is necessary to provide the bromide print with a 
“safe edge” by leaving a white margin of 4 to % inch all around the 
print. 

Where the water supply contains lime, it is well to place the print, 
after fixing and washing, in a solution of hydrochloric acid and water 
(3 parts concentrated HCl to 100 parts of water) for 5 minutes, then 
wash for ten minutes. If this is not done there is a danger that the 


. 


3 
# 
q 
; 
4 
y 


PROCESSES EMPLOYING BICHROMATE COLLOIDS 505 


lime formed within the bromide print will prevent complete bleaching 
of the highlights with the result that they wash away in the develop- 
ment of the carbro. - 

Should any spotting be required, or it is desired to darken certain 
portions in such a way that the result will be reproduced in the carbro 
print, one may use water color containing Indian ink. All such al- 
terations are reproduced in the carbro print with as complete trans- 
ference as any other part of the image. 

Sensitizing the Carbro Tissue——For this purpose the following 
stock solutions are required: 


Concentrated Solution No. 1: 


RCE SIME EUSNAUC cs a cc etsy eee cree es cccucs t_ Oz. 10 gin. 
OE a Te or ar T O02: 10 gm. 
Tg blogs 9 10-em. 
ee 20 OZ. 200 cc. 
Concentrated Solution No. 2: 
Re Sy NS Sn rr rr ae IO CC. 
PROT IG AGHIGN DULTE ) oc, ones ee ces we van cee ces I oz. IO cc. 
OE ENCE TSE GY Sad ele 2] Sc 22 02. 220. CC; 


The addition of 1% oz. or 12 cc. of water to the above will pre- 
vent precipitation in cold weather. 
For use take: 


First Bath: 


Ssoneemirated 1V0).4 stock Solution... ... 0.6.55 000023 6 oz. 100 cc. 

BEE SS ONE a ae 18 oz. 300 cc. 
Second Bath: 

Concentrated NG.-2 stock soltition:...:.¢........... I OZ. 10 Cte 

Us ev a 7 Sh a aes Se ee er 42-G2, 320 cc. 


The above English and metric weights are not equivalent but are 
proportionate. 

The first bath may be used for some time, fresh solution being 
added as its bulk grows smaller. The second bath, however, should 
be frequently renewed as it is altered by the No. 1 solution which is 
carried over into it. 

Both concentrated solutions keep well in tightly corked bottles away 
from light. The temperature of the working bath should be kept as 
nearly as possible at 60-65° F.—a lower temperature will lessen the 
activity of the solutions while a higher one will increase their activity 
and bring on various troubles. 


506 PHOTOGRAPHY 


Sensitizing the Pigmented Tissue—The working baths having 
been made up, the sheet of carbon tissue is immersed in No. 1 bath 
by sliding it under the surface of the solution. Remove any air-bells 
on the face of the tissue and then turn it face downwards. After two 
minutes the pigmented tissue should be again turned face upwards and 
allowed to remain for another minute. Then lift it up by the corner 
and let it drain for 15 seconds. Finally grasp it by two corners and 
slide it face up into solution No. 2. 

The time of immersion in this second bath is governed by the effect 
desired and by the brand of bromide paper employed for the print. 
It is impossible to give the best time for immersion since so much de- 
pends on working conditions, but as a rule the time ranges from 15-30 
seconds. The shorter the time of immersion the greater the contrast 
of the carbro print, while with longer times of immersion the less the 
degree of contrast. It is thus possible by varying the time of im- 


The upper drawing shows the hinging of the clamping strip at the 
requisite height above the base. In the lower drawing is indicated 
the edging of studded rubber. ‘ 


Fic. 222. Squeegee Board for Carbro. (Farmer) 


mersion in the second bath to produce a print having the same degree 
of contrast as the original, or one having increased or diminished con- 
trast. Thus if under a given set of working conditions 20 seconds im- 
mersion produces a carbro print identical with the original bromide, 
15 seconds immersion will produce a carbro having greater contrast 
and 25 seconds less contrast than the original. It is obvious that such 
control may be employed to control the gradation of the carbro print 
so as to secure exactly the brilliancy desired. 


PROCESSES EMPLOYING BICHROMATE COLLOIDS 507 


The number of seconds decided upon having elapsed, the pigmented 
tissue is lifted from the bath and laid upon the bromide print which 
should have been previously soaked in water for % hour and laid 
upon a sheet of glass or upon the squeegee board illustrated in Fig. 
222. Once the two are in contact their relative positions must on no 
account be altered as the action begins immediately and a change in 
position would produce a blurring of the image. As soon as the pig- 
mented sheet is in contact with the bromide print a rubber squeegee is 
brought into play and the superfluous water forced out by firm, 
straight strokes. 

The bromide print with its adhering sheet of carbon tissue is then 
lifted from the glass or squeegee board, placed between greaseproof 
paper and allowed to remain for 15 minutes for the action of the in- 
solubilizing solution to take place. 

Transfer.—In the meantime a sheet of transfer paper of the de- 
sired grade should be placed in cold pure water and allowed to soak 
for at least 5 minutes if thin, or 10 minutes if thick. Then, at the 
expiration of the fifteen minutes, strip the pigmented tissue from the 
bromide print, drop the bromide print in clean running water, and 
place the pigmented tissue on the transfer paper, squeezing the same 
so as to secure perfect contact. Finally place between blotting papers 
under slight pressure and allow to remain for 20 to 40 minutes. 

Redevelopment of the Bromide Print.—While this is being done 
the washing of the bromide print may be attended to and when thor- 
oughly washed it is redeveloped. For this a plain metol developer 
is advisable although metol-hydrochinon may be used. Care should 
be taken in any case that development is thorough and to this end it 
is well to leave the bromide print in the developer several minutes 
longer than would be judged necessary from its appearance. Fixing 
is unnecessary and after washing and drying the print is again ready 
for carbro printing. 

Development of the Carbro.—The development of the carbro is 
much the same as the development of a carbon print made by the 
older method. The transfer paper with its adhering sheet of pig- 
mented tissue, the latter uppermost, is placed in a large dish of warm 
water at a temperature of about 95° F. (35° C.). This is somewhat 
lower than the temperature found necessary for carbon and is due to 
the fact that the soluble gelatine leaves the image at a lower tempera- 


508 PHOTOGRAPHY 


ture and more rapidly than with a carbon print made in the ordinary 
way. One should not attempt to judge the temperature of the water 
by the finger but use a thermometer. In a few minutes the pigment 
will begin to ooze out around the edge; when this occurs, separate the 
transfer paper and the paper backing of the pigmented tissue and 
gently strip the latter off and throw away. If the pigment shows a 
tendency to stick to the tissue backing so that parts of the image are 
pulled up from the final support, the tissue is old and warmer water 
should be used for stripping. The paper backing having been re- 
moved, grasp the print by one corner and gently splash water over 
it with the other hand. As the pigment is very soft at this stage, on 
no account must the image be touched or treated with any violence 
whatever. If after a short while the print is still too dark, warmer 
water may be used. There is quite a little control possible in de- 
velopment by the use of colder or warmer water. It is best, however, 
to resort to this only when other agencies have been exhausted. 

If it is desired to lighten any portions this may be accomplished by 
pouring on such portions a thin stream of warmer water, taking care, 
however, that the force of the same is not so great as to wash up the 
image. By this means a highlight can be brightened or a heavy, 
blocked up shadow lightened so as to bring out buried detail. 

When development is judged to be complete, the print ‘is removed, 
raised in clear cold water and placed in a 5 per cent solution of alum. 
This removes the yellow stain left behind by the bichromates and 
ferricyanide and hardens the image. Care should be taken that the 
action of the alum is complete as the yellow stain is much more ap- 
parent when the print is dry than when wet. In commercial prac- 
tice it is well to use two baths of alum; immersing the print until ap- 
parently clear in the first, then transferring to the second for 3 or 4 
minutes. 

After removal from the alum bath the print should be rinsed well 
in cold water and hung up to dry. Heat should not be used to hasten 
drying. 

Carbon on Bromide.—lIf desired the pigment image can be de- 
veloped on the bromide print instead of transferring to a new paper 
support. The procedure is just the same except that when the fifteen 
minutes of contact between the bromide and the pigmented tissue have 
elapsed, instead of stripping off the pigmented paper, both it and the 


PROCESSES EMPLOYING BICHROMATE COLLOIDS — 509 


bromide print are placed in warm water and developed as already 
described. The print then consists of a pigment image over the 
bleached image of the bromide print. This last may be allowed to re- 
main, redeveloped or removed by means of the ordinary ferricyanide- 
hypo reducer. 

As the yellow color of the bleached image alters the tone of the 
finished print and since it darkens slightly on exposure to light it is 
advisab’e either to redevelop the bromide image or to remove it com- 
pletely. Redevelopment of the silver image darkens the print since 
in this case the resulting print has the depth of the two images; one 
of silver and the other of carbon. This property may be used to ad- 
vantage in dealing with weak negatives from which it is impossible to 
get sufficient richness in the ordinary way. 

Multiple Printing.—The first, I believe, to call attention to the 
simplicity of multiple printing by the carbro process was Paul L. 
Anderson in the American Annual of Photography for 1923, p. 44. I 
repeat his remarks on the subject: 


Multiple printing by the non-transfer method (carbro on bromide) is ridicu- 
lously easy, for whereas registration marks are necessary in the transfer ‘method, 
no such precautions are required in non-transfer; the second sheet of sensitized 
tissue is squeezed down on the redeveloped bromide and the silver image itself 
takes care of registration. The writer has never put more than three printings 
of carbon on one bromide, but there seems to be no reason why an indefinite 
number should not be applied if necessary: however, three will generally take 
care of any desired effect. 


It is obvious that multiple printing may be employed for the pur- 
pose of printing one color over another, or for increasing the range 
of gradation and adding to the finished print a quality which cannot 
be secured by a single printing. As was shown by Hubl as early as 
1898 in regard to the gum-bichromate process,’ the very best results 
are obtained from a long-scale negative when two or more prints are 
made and superimposed; one print being soft and the other contrasty. 
In this way is obtained ‘“‘a result which often surpasses, in truth and 
fidelity to the original, a normal print from the negative.” 


9 Eder, Das Pigmentverfahren, der Gumii-, Oel und Bromoldruck, Halle a/S, 
IQI7. 


34 


510 PHOTOGRAPHY 


GENERAL REFERENCE WoRKS 


BeLtin—Manuel Practique de Photographie au Charbon, 1900. 

BraHAM—The Carbro Process. 

Co_tsEN—Les Papiers Photographiques au Charbon, 1898. 

Eprer—Das Pigmentverfahren, der Gummi-, Oel-, und Bromol-druck und ver- 
wandte photographische Kopierverfahren mit Chromalzen, 1920. 

LieBERT—Photographie au Charbon, 1908. 

LiEsEGANG—Der Pigmentdruck, I9gII. 

Marton—Modern Methods of Carbon Printing. 

SAwYER—The A. B. C. Guide to Autotype Carbon Printing. 

STENGER—Die Kopierverfahren, 1926. 

STOLzZE—Katechismus der Chromatkopierverfahren, 1904. 

Sport—Der Pigment-Druck, 1920. 

VALENTA—Photographische Chemie und Chemikalienkunde, 1922. 

VocEL AND HANNEKE—Pigment-verfahren. 

Watit—Carbon Printing. 


Chiat PERS ATI 


THE GUM-BICHROMATE PROCESS 


Introduction.—The gum-bichromate process is another of the sev- 
eral processes which depends upon the fact that chromatized colloids 
become insoluble upon:exposure to light. It differs from carbon in 
that the colloid used is gum-arabic instead of gelatine and also that 
there is no transfer, the print being exposed and developed from the 
front, which renders multiple printing necessary in order to secure a 
full scale of gradation. Gum is without doubt one of the most flexible 
printing processes known and can far surpass any other in quality in 
the hands of a worker who understands it and knows how to get what 
he wants. Almost any degree of contrast may be obtained and local 
values may be altered to suit the artistic sense of the worker. As in 
carbon there is a wide variety of colors available and also an even 
wider range of surfaces from which to choose. The scale of grada- 
tion which may be secured by multiple printing is greater than any 
other process can render. Aside from these there is a quality in a 
eood gum print, particularly in the shadows, that makes it superior to 
all other printing mediums with the exception of photogravure and 
possibly the oil processes. The shadows can best be described by say- 
ing that they have depth and richness, without any gloss or muddiness. 
For the worker who takes a pride in the artistic quality of his work 
and whose desire is to turn out a few good prints rather than a great. 
many, gum is an ideal process. : 

Owing to the coarse grain of the gum coating, small detail is de- 
stroyed so that the process is only suited to broad subjects having no 
important detail—such as the artist calls “big” subjects. For the 
same reason, gum is at its best in large sizes, 8x 1o and larger. As the 


paper is not sufficiently sensitive for enlarging, an enlarged negative 


must be made and this is a handicap to many workers as is also the 

fact that exposure must be to daylight. A great deal of painstaking 

care and attention is required at each and every stage of the various 

operations of coating, exposing, and drying, and as these have to be 

repeated at least twice and often as many as five times in order to 

secure prints having the proper depth and quality, the process is a 
511 


512 PHOTOGRAPHY 


lengthy one and one which demands the energies of the worker for a 
longer space of time than many can spare from their other occupa- 
tions. 

Materials.—The papers which can be advised for the process are: 
“ Griffin” detail paper of Seltmann of New York City, Strathmore 
detail made by the Mittineague Paper Co., Whatmans, Michallet, Al- 
longe, Lallane and English cartridge paper, practically all of which 
may be had from paper dealers in the larger cities. Handmade paper 
is to be preferred to machine made, as it is tougher and has not been 
strained in manufacture so that the fibers do not all run in one direc- 
tion. 

The colors used are the moist water colors sold in tubes. The makes 
of Devoe and Windsor and Newton are to be recommended. For a 
beginning, one or two tubes each of ivory and lamp black will do, 
while the following seven colors: ivory, black, lamp black, Venetian 
red, chrome yellow and Prussian blue, will cover Praca all normal 
requirements. 

In addition to the paper and color, several trays about a size larger 
than the largest print will be required; also two or three graduates 
(about 8 and 16 ounces); a supply of glass-headed push pins; several 
ounces of granulated gum-arabic; two brushes, one a rubber bound for 
coating and the other a broad soft brush for bse and a board 
about twice the size of the paper to coat. 

The Negative.—The negative calls for little attention as owing to 
the personal control which may be exercised in printing, any variations 
in contrast can be made, according to the taste of the worker. How- 
ever, a thin negative seems to print better than a dense one, even though 
they may be practically identical in other respects. Of course, it is 
best to aim at a technically perfect negative having normal contrast and 
with minimum density, but by multiple printing the shadows may be 
printed in one operation, the half tones in another and finally the high- 
lights in a third, so that the final result is completely under the control 
of the worker. 


Formulas.— 
Gum SOLUTION 
Watered..oia ies Taha ae, eee 12 Oz; 1000 CC. 
Guu-atabies-.c ew. d's sivas k nceip ated oe 2200 gr 366.6 gm 
ALVOWMOOb ce iy.ricend se cs iho nh oe leg ae 270 gr 44.6 gm 


>? eye" 


ee ae. a a Fo we 


| 
| 
4 
| 
: 


THE GUM-BICHROMATE PROCESS 513 


Dissolve the mercuric chloride in a small amount of water and then 
add the arrowroot, stirring the same until a thin cream is obtained. 
Then add the remaining water and the gum-arabic. The latter will 
dissolve more rapidly if suspended in the solution by means of a cheese- 
cloth bag. From sixteen to twenty-four hours will be required for the 
latter to completely dissolve. 


SENSITIZING SOLUTION 


The stock solution of sensitizer consists of a solution of potassium 
bichromate : 


SL etl es le aa a a 1S oz. 1000 Cc. 
BOONE MIEN POMIALE 06. 6. cals ands vee ee en es 720 er: 96 gm. 


Both of these solutions keep well. 

The actual mixture used for coating varies with the paper and the 
negative and also with the effect desired. Practically every worker 
develops a different formula after practice and while there may be little 
difference, yet it is better adapted to his own personal methods of 
working. | 

However, the following formulas are given for the benefit of the 
beginner : 

SHADOW COATING 


(sum- solution... ...«. ae An Sek ba dy Y oz. 15 gm. 
eee TM i a DG bee wala weno 1 Oz. 15 gm. 
EEO) AINE. oc oc ih vs cans saa dio eve tee es atta 4 in 


If the negative has a short scale of gradation it may be possible to 
use the above for all of the printings; if, however, this is not the case 
and the negative has a long scale of gradation and prints well with 
bromide or platinum, then it will be necessary to vary the coating mix- 
ture so as to secure a longer scale. It is generally necessary to make 
three printings: one for the shadows, another for the halftones and 
finally one for the highlights. ‘The following are advised for the half- 
tone and highlight coating mixtures: 


HALFTONE CoATING MIXTURE 


emer AP SL ek Cha ce ga eb he ee © TY oz. ie) em, 


eR eE ee isl dice. ie aulee We Ca « 1% oz. 1S) ori: 
POE eC A POT TUDE fo ob na Sa es es wiv owes 260% 18: verte se 


HIGHLIGHT CoatiInGc M1IxTuRE 


SMES ok Falk oe ae hile an hoa wow ea dances ue oz. 1s gm. 
Sensitiger .i 6. ::.. sald 8 ae WERT SOARS gO ge CA Ge ar, | ae By a oo 


POPNOW? TUDE. ie ca ge de rec cee ede cee® ay ce ode 3 


514 ? PHOTOGRAPHY 


Effect of Varying Proportions of Coating Mixture.—Although the 
above may be regarded as an average formula, considerable variation 
is possible, but the beginner will do well to stick by the above until he 
is familiar with the process and knows what steps to take in order to 
secure the desired result. Increasing the amount of pigment gives a 
longer range of tones but the whites are stained and lack purity. An 
excess of gum makes the coating hard to blend smoothly and produces 
a thick film which may chip off in development. A moderate increase 
in the amount of gum solution gives greater contrast and the high- 
lights may be blocked. An excess of sensitizer gives a coating which is 
difficult to spread and one which gives flat, lifeless prints. A rough 
paper will take a thicker coating than a smooth one and is also more 
easily coated so that the latter is not to be recommended for the 
beginner. 

Figure 223 gives schematic curves for gum pigment with varying 
amounts of gum, pigment and sensitizer. Examination of the same 
will show that (1) has a long scale of tones with little contrast, (2) 
shows a shorter scale of tones with greater contrast, while (3) shows 
a short scale of tones with high contrast. ‘This gives an idea of the 


a) 


Maaazrez2 9 67 Ee eG OTE TF HDR 


Fic, 223. Curves Showing the Influence on Contrast of Variations in Propor- 
tions of Gum, Pigment and Sensitizer in the Gum-Bichromate Process 
(Anderson) 


immense variation which may be produced in a single printing by 
alterations in the composition of the coating mixture. Further varia- 
tions may be made by using different coating mixtures for the different 
printings and by local control, so for these reasons it may be readily 
seen that gum is justly regarded as one of the most flexible processes 
which we have for the production of positive prints. 

Coating.—The coating mixture having been prepared, the sensitiz- 
ing of the paper may proceed. Rough papers are the easiest to coat 
and the beginner is therefore advised to start with a rough paper, as 


et ee F ph oats i ar 
oe he ae Eee Ey ei en i ee a 


THE GUM-BICHROMATE PROCESS 515 


Whatman’s. The sheet should be larger than the negative as it is 
next to impossible to secure a perfectly even coating to the very edge 
of the paper. A sheet 11x14 will be suitable for either 8x Io or 
10 x 12, while an 8x Io sheet is sufficiently large fora 5x7. Attach 
the paper to the drawing board with push pins and pour the coating 
mixture in the center. The paper will be more easily coated if it is 
immersed in water and blotted before being placed on the board. For 
the first 11x 14 sheet of paper, about one half ounce of the coating 
mixture will be required, while succeeding sheets will require some- 
what less, owing to the brushes becoming charged with the gum. The 
rubberset bristle brush is used to spread the coating over the paper so_ 
that every part is covered with the coating mixture. When this has 
been done, the blender comes into play. There are two brushes which 
may be used for blending (one made of fitch and the other of badger) 
and the details of the operation depend somewhat upon which is being 
used. With the former the brush is held nearly vertical and drawn 
slowly and regularly across the paper—always in the same direction. 
When the sheet has been covered in one direction, it is again gone 
over in the opposite direction, to secure a perfectly even coating. 
With the latter, the action may be “ whippy,” the vigorous handling of 
the brush lessening as the operation proceeds. The exact manner of 
handling the brush and the time to stop blending will come with a 
little experience. In general, it may be said that when the point is 
reached where the tendency of the gum solution is to run into small 
puddles, the operation should be stopped whether the surface ap- 
pears completely even or not. Any irregularity will disappear in dry- 
ing or development or will be covered by subsequent printings. As 
soon as the operations are complete the utensils should be washed free 
from the gum solution, as it is very difficult to remove when dry. 
Drying.—The paper may be dried in a dim light, provided it is to 
be used as soonas dry. ‘The paper is insensitive when wet but becomes 
sensitive when dry. However, if the paper is to be stored for any 
length of time, it should be dried in the dark as the action of light con- 
tinues, and the gum will become completely insoluble unless it is dried 
ina dark place. The time of drying will depend altogether on the tem- 
perature of the room, but with an ordinary room temperature of 65 to 
75° F. (18-24° C.), the time required should not be over an hour or 
so. If not for immediate use, it should be placed in an airtight con- 


tainer, similar to platinotype, for storage. The paper is at its best 
when fresh.} 

Exposure.—As the image is invisible a photometer must be used 
to gauge the time of exposure. The photometers illustrated and de- 
scribed in the former chapter on carbon printing are suitable for the 
purpose and the serious worker should secure one of these. When 
the light is steady, the frame may be loaded with proof paper and the 
time required for reaching the desired depth noted and the gum paper 
exposed directly afterward for the same length of time. Since, how- 
ever, it is rare that the light is uniform, the use of an actinometer is to — 
be advised. There is a good deal of latitude in exposure but correct 
exposure will greatly simplify development and give the best results. | 
Over exposure is preferable to under exposure, as the development of 
the former may be forced by the use of hot water or an alkali, while 
the latter once in the developer is useless. If it is known that the 
print is under exposed, it may be laid away for several hours before 
development. The action of the light continues in the same manner as — 
in carbon printing and this method may, therefore, be used with ad- 
vantage when the light is dull and long times of exposure are required. 
But since it is difficult to determine the exact rate at which the action 
proceeds, it is preferable to expose fully and develop in the normal 
manner. 

Development.—One of the “talking points” for the gum-bichro- 
mate process when first introduced was the ease with which local values 
might be altered by the use of a brush or sawdust, etc., in develop- 
ment. While, to a certain extent, local work of this nature is now 
done, most workers now content themselves with automatic develop- 
ment. The exposed print is immersed face up in a large tray of cold 
water and as soon as limp turned face down, care being taken that no 
air bells are imprisoned beneath it, where it is left with an occasional 
examination, for one half to one hour. If the image is completely 
developed within ten or fifteen minutes the print is under exposed and 
may as well be thrown away. If the image does not appear within j 
one half to three quarters of an hour, the print has been over exposed __ 
and the temperature of the water may be raised slightly. The use of 
an alkali is not to be advised when multiple prints must be made, nor 
should the temperature of the developer be raised over go degrees. It 
is better to prolong the time than to raise the temperature or resort to 


516 | PHOTOGRAPHY 


1 Heat may be used for drying but not at a higher temperature than 180° F. 
GS 


+n 


THE GUM-BICHROMATE PROCESS 517 


an alkali in such circumstances. When the solution which drains from 
the print, when removed from the water, is practically clear, develop- 
ment may be considered complete and the print placed in a horizontal 
position to dry, care being taken that nothing comes in contact with its 
surface until dry. When dry it is ready for the sensitizing, exposure, 
etc., for the second printing. 

To lighten any local portions the print may be held under water and 
a stream of water from the tap-allowed to fall upon the desired por- 
tion, or an atomizer used. Greater emphasis may be secured by using 
hot water or by holding the print so that the stream of water falls di- 
rectly upon the surface. Local values may also be lightened by the 


_use of a very soft brush. ‘There is a tendency for brush work to show 


graininess and for that reason it should be avoided whenever possible. 

If it is desired to darken any part, the coating mixture may be pre- 
pared and applied by means of a brush to the desired portions and the 
whole exposed to light when dry. It is then washed in water for about 
half an hour and dried in the regular way. It is necessary to include 
the sensitizer in order to secure the same tone as the original deposit. 

Registration.—When making multiple prints it is necessary to em- 
ploy an accurate method of registering the separate printings so that 
they fall exactly over each other. Many methods have been devised 


GUM PRINTING FRAME 


SPR Sp 
5 oN 
5 


CORNER DETAIL iy 


sy 


Fic. 224. Owens’ Frame for Multiple Printing 


for this purpose and a few of the most generally useful will be de- 
scribed here. A very accurate and convenient method is that devised 
several years ago by Horsely Hinton for making combination prints. 
The sensitive paper, which must be larger than the negative, is placed 
face up on a smooth drawing board and the negative placed with the 


518 PHOTOGRAPHY 


emulsion side in contact, when necessary a thick piece of plain glass 
being placed over the whole to secure perfect contact. On one side 
of the negative two stout pins are fixed firmly in the board and two 
similar pins placed against the negative on the contiguous shorter 
dimension. Registration is secured by replacing the pins in the holes 
after each printing and forcing the negative up against them. 

James Owen in American Photography, 1923, p. 416, describes a 
printing frame designed by him especially for multiple printing. The 
construction is simple and obvious for the purpose in view, which is 


to provide a backboard on which to lay the sensitized paper either 


for the first printing or after one or more previous printings; dn the 
paper is laid the negative, and the glass panel is then clamped down 
by means of two simple wooden buttons, with minimum chances of 
disturbing the registered relation between negative and paper. 
Registration is accomplished by using the corners of the negative 
as reference points. When the first printing has been made, clean 
impressions of the edges of the negative are usually left. With a hard 
pencil the edge lines are prolonged as shown in the accompanying 
diagram, before applying the second coating. Whatever shrinkage 
the print has undergone its first or succeeding development is readily 
distributed as shown in the diagram. With some hard finished papers 
there is little or no shrinkage but the pencil lines serve for registering 
the corners of the negative exactly with the corners of the printed 


image. The essential point is that a frame of this type is a simple 


device in which print and negative may be quickly registered and 
clamped for printing without slipping out of register. 

The size of the frame naturally depends upon the largest size of 
negative to be printed from. It should be several inches larger, all 


round, however, than the largest negative to be used. Figure 224 will 


give a general idea of the principle and the details of construction. 

Mr. William H. Zerbe uses a frame several sizes larger than the 
negative with a sheet of plain glass, to this glass he attaches in one 
corner two strips of glass to form a true square. On the sides of these 
strips, about where the center of the negative will come, a piece of 
gummed paper is fixed. Lines are then drawn across in the exact 
center from side to side and top to bottom as shown in Fig. 225. 

The back of the paper is worked off with a T square, either before 
or after coating, making a line about 34 to 1 inch at the edges in the 
center of sides, top and bottom. In this way the marks are square 
although the paper may not be. 


THE GUM-BICHROMATE PROCESS 519 


The negative is then placed in position, one corner being forced into 
the frame made by the two strips of glass. The paper is then placed 
over the negative and the marks on the paper and the strips gummed 
on the glass made to coincide. After the first print is developed and 


Gi 


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Fic. 225. Zerbe’s Method of Registration 


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recoated, all that is necessary for registering is to make the registra- 
tion marks coincide again. Should there be any stretching of the 
paper it will be distributed four ways and minimized.? 

Gum-Bromide and Gum-Platinum.—To avoid the difficulties of 
multiple printing (which is necessary in the ordinary method of gum- 
bichromate in order to secure an image having the proper depth and 
gradation) while at the same time preserving the quality of the gum 
print, many workers have combined the process. with bromide or 
platinum, using a print made by one of the latter as a foundation print | 
to supply the needed depth to the shadows and half tones, over which 
a gum-pigment image is made to secure richness and quality of image 
which it alone will yield. 

Since the character of the finished print is greatly affected by the 
depth and contrast of the foundation print, the ultimate end should 
be carried in mind when making the bromide or platinum print. Gen- 
erally speaking, the contrast of the foundation print should be rather 
stronger than usual. The shadows, however, must not be as dark as 


<- | 


2 American Annual of Photography, 1923. 


520 PHOTOGRAPHY 


desired in the finished result, for it must be remembered that their 
intensity will be increased somewhat by the layer of pigment superim- 
posed over them. , 

The proper depth varies in exactly opposite ratio to the amount of 
pigment used in the gum coating :—the stronger the gum-pigment coat- 
ing, the lighter should be the shadows of the foundation print—and 


vice versa. If the intention is to use a gum coating weak in pigment, » 


so as to obtain just a slight glaze of pigmented gum, the foundation 
print must be almost as dark as the finished result is required, but, on 
the other hand, a strong pigment image only requires a weak founda- 
tion print sufficient to give the additional intensity required by the 
darker tones of the subject. It.is therefore necessary that one have 
definitely in mind the effect which is desired and proceed to make the 
foundation print accordingly. 

Before exposure the bromide or platinum paper, as the case may be, 
is placed upon the negative and its position registered by any of the 
‘means already considered. It is then exposed and developed in the 
usual way, to produce an image of the required intensity. When dry 
the print is ready for the gum-pigment coating. No definite rule can 
be given for the coating mixture as so much depends upon the strength 
of the foundation print and on the effect desired. The only way to 
determine the quantity of pigment to employ in any particular case 
is to spread some of the mixture upon a waste print of approximately 
the same character and note the effects. The operations of coating, 
exposure and development of the gum-pigment image are practically 
identical with multiple gum-bichromate and need not be repeated. 

The Powder Processes.—There is still another series of non-trans- 
fer pigment processes based upon the action of light on bichromated 


Fe ee ee eee ae Ve ae ee a 


EE ————— es as eee 


colloids. This series comprises those processes which are collectively 


termed the powder processes. In these an image is first formed in 
bichromated gelatine after which pigment in powder form is dusted 
over it. The pigment adheres to those portions of the image repre- 
senting the shadows and which consist of a soluble colloid, while it 
adheres only with difficulty, or not at all, to those portions which con- 


sist of a less soluble or insoluable colloid, and in this way the image ~ 


is produced. Of the large number of processes of this nature de- 


scribed in the older works, such as Abney’s /nstruction in Photog-— 


raphy, practically none have survived. Mr. E. J. Wall, however, has 


THE GUM-BICHROMATE PROCESS 521 


recently described a powder process which would appear to have a 
more promising future.* | 

The bromide print, which should have been fixed in plain hypo and 
not in an acid fixing and hardening bath, is transferred directly from 
the last wash water to the following solution : 


MRT IUACE Ye peg os ik eon he ee oe a ee ee ws a 5 5 gm. 
eG MOG i vhs se ee he ea a eee aes 45) mini” Gir cee 
ce al ee ee: 0.5 gm. 
TCE OEE ONICG. gcd ihe ve oven sc oe ene aaes 196 f..0z., 100.cc 
REF 2 se eve a vials lanl a's wieeee ae 165.07. 1000 cc. 


which should be used at a temperature of 70° F. (21° C.). The 
duration of the action ranges from 5-20 minutes, being dependent 
probably on the emulsion. 

For the powdering of the image any inert pigment, black or colored, 
may be used, but it should be as finely ground as possible, or the re- 
sults may show an undesirable amount of grain. The powder may be 
applied either with a very soft brush or by means of a little sieve hav- 
ing a bottom of the finest muslin. Some of the powder is placed in 
the sieve which is held over the image and tapped with the finger. 
This method is perhaps preferable when using a pigment containing 
rather coarse particles. 

Resinopigmentype.—Resinopigmentype, a method worked out by 
Prof. Rudolph Namias, belongs to the same class of printing processes 
which we have just discussed. It has attracted considerable attention 
among pictorialists on the continent and Mr. Joseph Petrocelli of New 
York has produced some very beautiful work by the process. 

It is especially adapted to subjects which do not have a high degree 
of contrast, as, for instance, winter scenes, the effects of fog, rain, etc. 
On the other hand, it is ill adapted to images requiring vigor and con- 
trast, for it is impossible to obtain absolutely pure whites. | 

The point of departure of the Resinopigmentype process is in the 
use of a positive transparency, which may be on glass, film or paper. 
This may be made from the negative either by contact or enlargement 
and must not be excessively contrasty. 

The paper supplied by Professor Namias is sensitized by immer- 
sion of the sheet of paper for three minutes in a 5 per cent solution of 
potassium or ammonium bichromate and drying in absolute darkness. 
It is preferable to sensitize in the evening, then the paper will be ready 


3 Amer. Phot., 1924, p. 428. 


522 PHOTOGRAPHY 


for use the next day. Nevertheless, the positive paper may be kept 
for one week in winter, or two or three days in summer, but the best 
results are obtained with the freshly sensitized paper. 

The paper is printed behind the positive in the manner usual with 
daylight printing papers, and the exposure is continued until there is 
a faint brown coloration under the transparencies of the positive, with 
the details of the half-tones lightly visible. Over exposure is to be 
avoided. | 

The most simple and effectual method of raising the relief is to 
leave the print film side down in a bath of cold water for several hours 
to eliminate the excess of bichromate. After soaking, the sheet of 
paper is placed in water at 50° C. (122° F.) for from two to five 
minutes, which produces a distinct image in relief. 

If time presses, the print may be swelled quite rapidly by plunging 
direct in water at 37° C. (98.6° F.), to which % per cent of ammonia 
has been added. After rinsing in cold water the print is ready for 
powdering. 

The excess of moisture on the swollen surface is removed with 
blotting paper or chiffon, but not with shaggy cotton or wool. One is 
then ready to begin powdering by means of a soft brush of polecat 
hair of medium size, dipped in the powder especially prepared for the 
process. On continually passing the brush over the paper, the image ~ 
appears and this operation is continued until the image is sufficiently 
vigorous. When necessary to remove any excess of powder, use a 
fresh brush. 

If the image obtained is deelents in contrast, it indicates that insuf- 
ficient powder has been applied, and in this event the proof is placed 
in a tray of cold water to detach all the powder and a higher relief if 
produced by ammonia water as previously recommended. 

The soaking in water advised is often insufficient to remove the last 
traces of bichromate, especially if the rapid method of swelling al- 
ready indicated is employed. The yellow stain is easily removed by 
immersing the print, before swelling, in a 10 per cent solution of 
sodium bisulphite or a § per cent solution of potassium metabisulphite. 
It is necessary to do this before powdering, because it removes the 
same when applied to the print. 


THE GUM- BICHROMATE PROCESS | 523 


GENERAL REFERENCE WorKS 


Brexnrens—Der Gummidruck, 1912. 

DemacHy AnD MaskrLtt—Photo-Aquatint or the Gum Bichromate Process. 

DEMACHY AND MAsxkeL_~t—Le Procede a la Gomme-Bichromatee ou Photo- 
Aquatinte, 1905. 

Eper—Das Pigmentverfahren, der Gummi-, Oel-, und Bromol-druck und ver- 
wandte photographische Kopierverfahren mit Chromalzen, 1920. 

HaNNEKE—Das Pigmentverfahren, 1912. 

von HormMerIsteR—Der Gummidruck, 1907. 

KosEt—Der Gummidruck, I9oo. 

Koset—Die Technik des Kombinations Gummidruckes un des Driefarben- 
Gummidruckes, 1906. 

Kosters—Der Gummidruck, 1904. 

Mayver—Der Gummidruck. 

QuEDENFELDT—Die Praxis des Geninidruck- verfahrens, I9QIo. 

RicHArps—The Gum-Bichromate Process, 1905. 

’ STENGER—Neuzeitliche photographische Kopierverfahren. 

STENGER—Die Kopierverfahren, 1926. 

WarreEN—A Handbook to the Gum-Bichromate Process, 1808. 

ZIMMERMANN—Zimmermann’s Method of Gum-Bichromate. Photominiature, 
113. 


CHAPTER XXIV 
THE OIL PROCESSES 


Introduction.—Oil and its companion, bromoil, are now two of the 
most widely used of all pictorial printing mediums. ‘This is due, with- 
out doubt, to the enormous flexibility of the process and to the ease 
with which the artist can alter values of the original negative to secure 
the particular effect he desires. The photographer has absolute con- 
trol over his result as any part of the picture may be darkened, light- 
ened, or even omitted at will. No other process offers the same facility 
to quite the same extent, although gum is a serious rival. | 

In the oil process, paper is coated with gelatine, sensitized with po- 
tassium or ammonium bichromate and allowed to dry in the dark. 
When dry it is sensitive to light and is exposed under the negative in 
the’ same manner as platinotype. When exposure is complete the 
print is placed in a bath of water in order to eliminate the bichromate 
stain and to allow the image to swell. In the bath the print gradually 
takes on a relief which is more pronounced in the strong highlights, 
since these have been more completely protected from the light and are, 
therefore, more soluble in water. The print is then removed and 
inked up with pigment applied by a brush. The shadows, owing to 
the fact that they have absorbed little or no water, readily take up the 
ink from the brush, while the highlights only take the ink with diffi- 
culty. Thus the image appears under the action of the brush and is 
gradually worked up to the desired depth by the application of ad- 
ditional pigment. The use of a hard pigment increases contrast, while 
thinning down the ink with medium causes the ink to adhere more 
easily and reduces contrast. The effect is also dependent upon the 
manner in which the brush is handled and this places an added means 
of control in the hands of the worker. 

Materials for the Oil Process.——The materials for the oil process 
are few in number and comparatively inexpensive. A good negative, 
one that has been properly exposed and has sufficient contrast to make 
a good bromide print, should be selected for the first attempts. While 
an experienced oil printer can secure a fair print from any reasonable 


negative, the beginner is advised to select a first rate negative as pig- 


524 


a 


THE OIL PROCESSES 525 


menting will then be easier and the result more likely to be successful. 
Aside from the negative and the paper, brushes and inks, which will 
be discussed at some length subsequently, the worker will need a solu- 
tion of potassium or ammonium bichromate for sensitizing, a supply 
of blotting paper, pallette knife, several pieces of glass about 5 x 7 and 
megilip or medium for thinning the inks. 

Papers for the Oil Process.—There is. no doubt of the fact that a 
great deal depends upon the selection of a suitable paper. While 
there are no important differences in any of the papers that are suit- 
able, some workers have better success with some papers than others, 
owing no doubt to a personal difference in manner of inking and the 
effect desired. for the beginner, the best advice that can be given is 
to select one of the papers named and stick to it until he is sure of 
himself. Then he may try other papers and experiment until he finds 
if any other suits him better. | 

The original Rawlings paper is supplied by Messrs. Griffin, of 
_ Kingsway (Kemble Street Corner), London, England. It is an ad- 
mirable paper in every respect and is one of the best papers that the 
beginner can use. It is made in smooth and rough and in sizes from 
3% x 4% to 16/18. As compared with other papers, the price is 
rather high. , 

The Autotype Company of London also issue two papers, No. 1 and 
No. 2, for the oil process. No. 1 is a smooth white paper; No. 2a 
toned paper with a fine grain. It pigments easily and stands vigorous 
brush work well. The final carbon support for double transfer is also 
used by some. Double transfer papers which can be advised are the 
Autotype papers Nos. 76, 77, and 9o. 

Several double transfer papers manufactured by Illingworth have 
been recommended by several workers, notably Demachy and Namias. 
Nos. 125 “ Thick Smooth” and 117 “ Thick Rough” can be recom- 
mended as suitable.* 

Brushes.—The brushes employed are especially made for the proc- 
ess. They are made from fitch hair and were formerly made only in 
France but are now also made in England. They are made of short 
spring hair and the end is cut at an angle. 

The quality of the brushes employed has a direct bearing on the 
finished result and only brushes of the best quality should be purchased 
even in the beginning. It is useless to try to get along with brushes 


1 Instructions for coating paper with gelatine will be found on page 21 of 
The Oil and Bromoil Processes by Mortimer and Coulthurst. 


35 


ME at We Fe he ot 


526 PHOTOGRAPHY 


made for other purposes and, while good brushes are rather expensive, 
they last a long time if kept in good condition and their purchase is a 
distinct economy. To begin with, three of these brushes will serve. 
These three may be Nos. 14, 10 and a small one for detail work. A 
larger “ Prima” brush, which owing to its being made of hog hair is 
cheaper, may be used for preliminary pigmenting and will be well 
worth its cost. As the worker progresses, it will be well to purchase 
additional brushes in order that he may lay down a charged brush and 
take up a new one when it is desired to apply ink of different con- 
sistency to any part of the print. Mr. F. J. Mortimer, who is one of 
the best authorities on the process, states that the following brushes 
cover all the requirements of the most advanced worker: © 


2 No. 14 Stag-foot Fitch brushes 
2 No. 10 Stag-foot Fitch brushes 
2 No. 7 Stag-foot Fitch brushes 
1 No. 5 Stag-foot Fitch brush 
t No. 10 Straight top brush 

1 No. 5 Straight top brush 


Brushes should be kept in good condition, not only because they are 
expensive, but because the quality of the work depends, to a large ex- 
tent, upon their condition. When pigmenting is complete the brushes 
should be completely cleaned and not allowed to become dry; when this 
occurs it will be practically impossible to remove the hardened pigment 
without destroying the good qualities-of the brush. Soak a clean rag 
in gasoline and rub the end of the brush on the rag. This will com- 
pletely remove any pigment adhering to the ends of the hairs but if 
the brush has been allowed to become clogged and the pigment is spread 
up within the brush, it will be necessary to soak the same in gasoline 
and finally wash out in soap and water until the brush is absolutely 
{ree from both pigment and gasoline. Take care not to get the brushes 
out of shape while cleaning and when they are thoroughly cleaned 
wrap them in a piece of white paper and place a rubber band around ~ 
the handle in order to keep the brush in its proper shape. 4 

Pigments.—Pigments are made especially for the process and are 
almost entirely of English and French manufacture. They are thick, — 
stiff, greasy inks very similar to those used for lithography, and the — 
latter may be used for the oil processes, but on the whole it is better to 
purchase that made especially for the process. There are several — 
excellent pigments on the market. The “ Rawlings” pigments are ~ 


ee a 


Tee EE ROGESSES 527 


excellent and Sinclair’s “ Permanent ” inks, as well as Roberson’s, may 
also be recommended. The Ault and Wilborg Company of Cincinnati 
will make ink for the process upon special order, which costs about 
half of the foreign product. 

A large variety of different colors can be obtained but black is recom- 
mended for the beginner, as it suits almost any subject. 

Sensitizing.—There is no necessity of varying the concentration of 
the sensitizing solution of bichromate for different classes of negatives, 
as is the case in carbon printing, for the final result is under complete 
control in the operation of pigmenting. Therefore, it is better to 
select a reliable sensitizing formula and make it the standard. Weak 
or thin negatives will be difficult to handle no matter what sensitizer is 
employed. The following formula is recommended by Mr. F. J. 
Mortimer : 


ya vu stad ae vet ceblwcavsasubwan 10 oz. 


For use take one part of the bichromate stock solution and two parts 
of methylated spirit. Care should be taken to thoroughly mix the two. 
The potassium salt cannot be used with methylated spirit as the latter 
precipitates the salt. Potassium bichromate, however, may be used 
with acetone in place of methylated spirit in order to secure a quick 
drying sensitizer and the following formula can be recommended : 


I part of acetone to each part of potassium bichromate (saturated 
solution). 


Many other formule are recommended by different workers but 
there is no particular advantage over those which have been given ex- 
cept that some of them keep longer when mixed ready for use. 

The paper may be sensitized either by immersion or brushing. The 
latter method is better since it is quicker and cleaner but paper so pre- 
pared does not keep so well as that coated by immersion and is best 
used very soon after it is dry, although it will remain in fair condition 
for twenty-four hours. Paper that has been sensitized by immersion 
will keep three or four days. Sensitizing may be conducted either in 
ordinary artificial light or weak daylight but should be removed to a 
perfectly dark place to dry. No gas should be burned in the rooms 
used for drying the paper. The paper must be thoroughly dry before 
use. 

To sensitize by immersion, sufficient sensitizing solution is poured in 


MS 
er x 


528 PHOTOGRAPHY 


a tray to cover the bottom to a depth of half an inch. The paper is 
placed with the gelatine side upon the surface of the solution and 
allowed to remain two or three minutes, removing it at intervals in 
order to break air bubbles. Drain the paper and hang up to dry. A 
Blanchard brush is the most convenient and practical brush for sensi- 
tizing. This consists of a piece of glass with fluffless flannelette 
wrapped over one end and secured with a rubber band. Using this 
brush the paper may be pinned to a board and sensitized in much the 
same manner as gum-bichromate paper ; the operation is much simpler, 
however. | 

Exposing.—The operation of printing is similar to any other print- 
ing process as P-O-P or Platinotype. The paper is about four times 
as fast as the former and slightly faster than the latter and care must 
be taken not to allow actinic light to reach the same in loading the 
frame or while examining the progress of printing. The image is 
semi-visible, in this respect closely resembling Platinotype. Printing 
is continued until detail is visible in the highlights. A few trials will 
show the proper stage to print. A photometer is not really necessary 
but may be a help where it is desired to make several prints as nearly 
alike as possible. 

It is advisable that the sheet of sensitized paper be at least an inch 
larger than the negative, in order that the print may be inked to the 
edge without danger of getting moisture from the inking pad on the 
brush. 

There is a slight continuing action after exposure and where several 
prints have to be exposed before pigmenting the prints should be im- 
mediately placed in a tray of water in order to stop the continuing 
action. | 

After exposure the print is immersed in water to eliminate the bi- 
chromate stain and to produce the necessary relief. Paper that has 
been sensitized by brushing will not take so long to become free of the 
bichromate as that sensitized by immersion. The water should be 
changed frequently or running water may be used. From one to two — 
hours will be required to produce the degree of relief necessary for 
pigmenting. The exact time will depend upon the climatic conditions 
and the temperature of the water. Warm water will produce a high © 
degree of relief very quickly but there is a danger of its affecting the © 
gelatine and causing the half-tones to be lost. Moderately warm — 
water, say about 75 or 80° F., however, may be used. Demachy has — 
suggested that a very small amount of sodium bisulphite be added to — 


J 


ee eo oer, a 


| 
| 


ee eee a 


Peo PROC SSES | 529 


the first few washing waters in order to facilitate the quick and thor- 
ough removal of the bichromate. 

After washing the print may be dried and pigmented at some future 
time or it may be placed on the inking pad and the operation of pig- 
menting begun at once. In the former case, it will be necessary to 
soak the print in water for about an hour in order to raise the relief 
and get the print in a suitable condition for pigmenting. If it is de- 
sired to pigment at once, the print is laid upon the wet pad to be de- 
scribed and the surplus moisture taken off with a blotter or, better, 
with a ball of silk or flannel. : 

Pigmenting.— This is the most important stage of the process. Un- 
fortunately, it is very difficult to give any precise information upon this 
point since it varies for every worker and methods which may be per- 
fectly adapted to one individual may be utterly useless with another. 
It is an operation in which the worker must develop his own methods 
and where his own skill and individuality must find the way. Never- 
theless, it is hoped that the few particulars which follow will be of 
assistance to the beginner. 

It is necessary to keep the print wet from underneath during pig- 
menting and for this a pigmenting pad is used. Pads are a commercial 
article and may be obtained from any of the dealers carrying oil ma- 
terials or one may be improvised from a sheet of glass and four or five 
sheets of blotting paper. Soak each sheet of blotting paper in water 
until thoroughly wet and then place on top of one another upon the 
glass plate. Upon this spread one or two thicknesses of muslin or 
cheesecloth. Then remove the print from the wash water and place 
on top of the pad. With a blotter or piece of silk take off the excess 
moisture, being careful not to make the surface wholly dry. Only take 
off the excess water. It will be noticed that the image stands out in 
relief and the appearance of the print at this stage is a good indication 
of the way it will take the pigment. The greater the relief the more 
readily the ink will take. 

Squeeze out a very small quantity of pigment on the palette. Only 
a small quantity is needed, as a piece the size of a pea will do for sev- 
eral8x10prints. Instead of a palette the glass side of an old negative 
may be used. Spread the pigment out in a thin layer with the palette 
knife and tap the brush on the same so as to take up the pigment on 
the end of the hairs. When the brush has become charged with the 
pigment, work it around on a clear portion of the glass in order to 
distribute the pigment evenly. It is well to prepare pigments of two 


530 | PHOTOGRAPHY 


consistencies at the beginning, since it is rare that the same pigment 
can be used throughout the operation owing to the fact that some parts 
of the image require a softer pigment in order to make the pigment 
adhere. Either linseed oil, Robertson’s medium or megilip may be 
used to soften the hardink. Only a trace of any one of these is neces- 
sary to completely change the ink and, therefore, they should be added 
with caution. 

As the image is only faintly visible, a straight gaslight print will be 
of service in indicating what should be done and if this print has been 
“worked up” in the same manner as the oil print, then the worker 
will have a clear and definite idea of the alterations to make in order 
to secure the results desired. 

The proper method of holding the brush is illustrated in Fig. 226. 
While the manner varies somewhat with the worker, the above may be 


Fic. 226. Proper Position of the Brush in Pigmenting. 
(Mortimer and Coulthurst, The Oil and Bromoil Processes) 


‘ 


taken as a safe position to use. The brush must be held lightly and 
not gripped. Nor should it be held close to the hair. The position 
is very similar to that adopted by the painter and if one has a friend 
who is a painter, he can no doubt secure some advice from him on this 
point. 

The method in which the brush is handled to apply the pigment 
also varies greatly as almost every worker has developed individual 
methods in practice. Some have a pressing, smudging action while 
others simply dab the brush on the surface. It is difficult to give any 
precise directions on this point and the worker will develop a dis- 
tinctive method of his own with practice. 


It is well to begin pigmenting with the ink as it comes from the tube : : 


and continue until the image is distinct and the shadows well defined, 
when it may be advisable to change to a softer pigment. It is easy to 


ee ke Pee: 


THE OIL PROCESSES 531 


tell when more pigment is needed for the brush will begin to pick up 
the pigment instead of depositing it. Use the hard ink as long as pos- 
sible for it is possible to apply softer ink over the hard if the latter 
refuses to adhere but hard ink cannot be applied over soft ink. 

To lighten any portions which have been over-pigmented the opera- 
tion known as “ hopping ” is used. In this the brush is held vertically 
above the surface and allowed to fall upon the paper. The brush is 
never dropped from a greater height than about two inches or the 
gelatine might be punctured and the print destroyed. Wire holders 
are supplied to hold the brush so that hopping may be carried on with- 
out fatigue. The operation should be looked upon as a corrective only 
and not used unless necessary as far better results are obtained by 
straight pigmenting to the point desired, but there are cases where it 
will be necessary to ink over small details and use the hopping action 
afterward to clear out the highlights so the worker should become — 
familiar with the operation. 

It is necessary that the print be kept in a moist condition through- 
out pigmenting or the pigment will adhere all over. If the side of 
the print underneath feels dry to the touch the pad should be re- 
wetted and the print may also be placed upon the surface of a tray of 
water for a few minutes and, finally, again removed to the pad, and 
pigmenting begun. 

Success in oil printing is dependent on two things—understanding 
and practice. The worker must not be discouraged if his results at 
first fail to satisfy but must stick it out and he will gradually note im- 
provement in the results. His first endeavors should be directed 
towards securing a smooth, even application of the pigment and be- 
coming familiar with the results secured with different inks and man- 
ner of handling the brush. After he feels that he has mastered the 
technical principles involved and can make a good straight print with 
certainty, he may then attempt to make alterations as his artistic taste 
may direct. Local values, however, should be studied very carefully 
and alterations should not be made until the worker has satisfied him- 
self that they are advisable. It is not exceedingly hard to master the 
oil process technically but to master it artistically is an achievement 
and one deserving of the reward which such masters as Demachy, 
Mortimer, Job and Puyo have received. 

Incorrect Exposure.—In the case of over exposure, the print takes 
up the pigment too easily and the shadows soon block up and lose 


532 PHOTOGRAPH. 


their details while the half-tones appear smudged and the highlights 
take the pigment also and rapidly darken. Ii the print is much over 
exposed, it may as well be thrown away, but if only slightly over ex- 
posed, the use of a hard ink and hopping may produce a passable re- 
sult. It is better for the beginner to throw away a print of this na- 
ture and make another as all the skill of an advanced worker is re- 
quired to get a passable print from one which has been incorrectly 
exposed. : 

With correct exposure, the shadows take the pigment gradually and 
the half-tones and highlights keep their proper relations. ‘This shows 
that the print has been suitably exposed for the pigment in use and 
all that is necessary is to keep on applying the ink until the desired 
result has been secured. Success in pigmenting depends greatly upon 
the exposure and every care should be taken to secure correct ex- 
posure in order that pigmenting may be a straightforward and certain 
operation. 

When the print has been under exposed there is difficulty in making 
the pigment adhere even in the shadows. Prolonged brush action 
causes the deposit to become granular and thin. This granular ap- 
pearance is always an indication of either under exposure or the use 
of too hard anink. The addition of a small amount of megilip or oil 
may be sufficient to soften the pigment so that it will adhere. If the 
first addition is not sufficient more may be added but it is better to 
use a hard ink than a soft one as the latter does not preserve the con- 
trasts properly but tends to produce the effects of over exposure. 
Only a very small quantity of oil or megilip is necessary to soften - 
the ink and care should be taken not to make it too soft. As in the 
case of over exposure, a print which has been very much under ex- 
posed is unsuitable and should be thrown away. - 

Drying and Mounting.—When pigmenting is finished the print 
may be hung on a line in a dust-free room to dry. It may be placed 
in a horizontal position for drying but in this position it is more likely 
to collect dust. The paper and gelatine may require two or three 
hours to dry but the pigment takes quite a time and it is well to allow 
at least twenty-four to thirty-six hours for drying. For mounting, it 
is best to use the dry process but the pigment should be thoroughly 
dry before mounting or it will come off while in the press. If glue 
or paste is used, it is best to only tip the corners so that they stick to 
the mount and not to try and mount the print flat. The print may be _ 


; 
| 
; 


ey en en eee Se ee 


ee ee ee ee Cae ae eee a ee a ee ee 


Pe a ee oS ee AT 


fie ho 9 aii 
aa See ea oe 


2 ees | ey pe OR 


THE OIL PROCESSES 533 


rubbed with a soft cloth after the pigment is dry to remove any par- 
ticles of dust adhering, but so far as possible these should be avoided 
by drying in a perfectly clean and dustless place. The point of a 
sharp knife may be used to remove loose hairs, etc., which are em- 
bedded in the pigment. 

Duvivier’s Process.—Monsieur Duvivier in his work Le Procédé a 
’Huile en Photographie describes a new process of oil printing in 
which the usual gelatine paper is replaced by one with starch. A thick, 
unsized paper is coated with starch, sensitized and exposed in the 
same general way as usual in oil printing. After exposure and de- 
velopment, the print is dried. It is then placed face up upon a pad 
of wet blotting paper. The paper is able to absorb water from the 
back, but those portions which represent the shadows and half-tones 
of the image are protected to varying degrees by the bichromated 
starch coating and remain dry while the highlights and lighter tones 
take up water in varying proportions. The highlights thus become 
moist enough to repel the ink while the shadows being dry take up 
the ink readily and the print is thus in a similar condition to the swol- 
len gelatine used in the usual oil process. In the case of the starch 
process, however, the difficulties of pigmenting are lessened owing to 
independence of the variations in the degree of swelling and conse- 
quently the adjustment of the pigments does not require to be as fine, 
so that the process is much simpler than oil. For full details the 
original work should be consulted. 

The Bromoil Process.—Very similar to the oil process is Bromoil. 
The bromoil process, in brief, consists in the making of a good bro- 
mide print in the ordinary way and bleaching this in a solution which 
produces an image in insoluble gelatine having the property of taking 
up pigment from a brush in just the same way as oil printing. Ow- 
ing to the fact that an ordinary bromide print is used, no daylight is 
necessary at any stage and as enlarged negatives are not required 
when an oil print larger than the original negative is desired, the 
bromoil process is a very popular one among pictorial werkers and 
bids fair to entirely supplant the older oil process. 

The Choice of the Paper for the Bromide Print.—While theoreti- 
cally any bromide paper should be suitable for bromoil, in practice 
such is not the case. There are considerable differences amicng va- 
rious papers in respect to adaptability to bromoil, while there are some 
few papers which can be used only with difficulty. The qualities of 


A ae 


534 Si PAOPTOURAT Eas 


a bromide paper adapted for bromoil as indicated by Professor 
Namias are: : 


1. Hard, durable and well-sized paper base. 
2. Emulsion rich in silver and gelatine and thickly coated. 
3. No hardening substances to be added in manufacture. 


It is not possible to use papers the swelling power of which has been 
lessened by hardening with alum, or other means, in the process of 
manufacture. The principle of the bromoil process is that a tanning 
of the gelatine shall take place differentially in exact proportion to the 
opacity of the original silver deposit, so that we get a tanned image 
in a bichromated colloid. -If, however, the emulsion has been hard- 
ened in manufacture the gelatine is already tanned and has lost most 
of its swelling power, so that it is impossik‘te to get the degree of re- 
lief necessary for proper pigmenting. | 

To determine whether a particular brand of bromide paper is suit- 
able for bromoil, Dr. Emil Mayer, in his Bromoil Printing and Trans- 
fer, suggests that an unexposed sheet of the paper be dipped in water 
at a temperature of 86° F. (30° C.) and the behavior of the gelatine 
film observed. If this swells up considerably and becomes slippery 
and shiny, the paper has the necessary swelling power and can be 
used for bromoil. 

A smooth matt paper is the best adapted for bromoil. Glossy papers 
are unsuitable, and there is, in many cases, difficulty with the rough 
surfaces of certain brands of paper. While most of the reputable 
brands of bromide paper may be used successfully, several manu- 
facturers now supply papers made especially for the bromoil process. 
These withstand rough treatment better, and being more thickly coated 


and unhardened, give more relief than ordinary bromide papers (the © a 


emulsion of which is nearly always partially hardened in manufac- 
ture), and are consequently more easily pigmented. Among such 
papers available at present, the following may be mentioned: Welling- 
ton Bromoil, Vitegas for Bromoil, and Gevaert for Bromoil. 

The Production of the Bromide Print.—It cannot be too strongly 
emphasized that the production of a bromide print suitable for brom- 


oil is a matter of great importance and one on which the success, or 


otherwise, of later operations largely depends. In fact, one should 


not attempt bromoil until he has complete mastery over bromide print- _ 3 ) 
ing and can make it responsive to his demands. The bromide print 


‘T=? = > 


———— Ee ae —— ee ee 


THE OIL PROCESSES | 535 


to be used for bromoil should be the best which that particular nega- 
tive will produce. The best prints for bromoil are the result of cor- 
rect exposure and development for a period slightly less than that re- 
quired for full depth. To this end the factorial method of develop- 
ment may be used as indicated in the chapter dealing with bromide 
printing. A lower factor should be used, however, and Dr. J. B. T. 
Glover recommends the use of a factor of 5 with the following amidol 
developer, which is the standard formula of the Kodak Company : ° 


Penn oy Oe ow vase ne eee ces 26.2 gr. .6 gm. 
Benmore nnitestdry) osc. eed es el TIO. gr. 25 gm. 
Potassium bromide (1 per cent solution)....... 7. eatin. 15 cc. 
em ie RSP, ls oy apn die cs oi are WA Riess 10). OZ, 1000 cc. 


Practically all developing agents in general use have a more or less 
pronounced tanning action on gelatine. The use of such developing 


_ agents, therefore, has the effect of producing an additional tanning 


action, not only on the shadows, where it might be desirable in certain 
cases, but also on the highlights where tanning of any kind is ob- 
jectionable. ‘The use of a developer with a pronounced tanning action 
has, in fact, the same effect as general fog in negative making. With 
emulsions which have been hardened in manufacture the use of tan- 
ning developing agents obviously is even more objectionable than in 
other cases. Accordingly the use of a developing agent without tan- 
ning action on the film is desirable. Such agents are amidol (Diamido- 
phenol), glycin and the iron developer. While the last named is, with 
certain precautions, excellently adapted to the development of bro- 
mide papers for ordinary purposes it is unsuitable for prints to be 
used for the bromoil process. Glycin is not especially well adapted 
to the development of bromide papers as it is slow in action and of 
the three, amidol is indisputably the best. There is no especial virtue 
in formulas and that given above will answer any requirement. The 
worker, however, may use that advised by the manufacturer provided 
development is regulated properly. 

The fixing which follows development is an important operation. 
A plain bath of 20 per cent hypo should be used. This must be made 
fresh for each bath of prints and discarded after use. The use of an 
acid fixing and hardening bath is to be avoided as, owing to its action 
on gelatine, considerable difficulty is experienced in bleaching and in 
securing the necessary relief for pigmenting. 


a, J.-F ., 1021, 68, 87. 


536 PHOTOGRAPHY 


Washing should be thorough, as the slightest trace of hypo left in 
the print will cause trouble in bleaching. 

At this point, before leaving the subject of the ieonikde print, it is 
well to remark that the print should have a plain white margin of at 
least half an inch. 

Bleaching of the Bromide Print.—As soon as washing is complete 
the bromide print may be bleached, or it may be dried and kept for 
bleaching and pigmenting at some future time. It is perhaps prefer- 
able, however, to allow the print to dry at this stage. Then when 
ready for bleaching it can be immersed in water for a few minutes 
until thoroughly limp. 

The functions of the bleaching solution are two in number: 

(1) It removes the visible silver image and (2) it causes a tanning 
of the gelatine film corresponding to the silver image that disappears. 
In place of the original image of metallic silver, there then exists an 
invisible one of differentially tanned gelatine. Ordinary reducers are — 
therefore unsuitable. They dissolve the silver image but do not pos- 
sess the property of tanning the gelatine film in the required manner. 
Dividing bleaching and tanning agents for the bromoil process into the 
substances they contain we have: 


a. Copper sulphate, potassium bromide, potassium bichromate. 
b. Copper chloride or sulphate, potassium bromide, chromic acid. 
c. Copper’ chloride, sodium chloride, potassium bichromate. 


No reliable methods of testing being available, it is impossible to say 
that any one of these is better than the other. Bleachers of all three 
types are used by various noteworthy exponents of the process and so 
much depends upon a knowledge of the bleacher and its action, and 


the mode of pigmenting, that it is in manipulation rather than in the — q 


type of bleaching solution that the causes. of failure should be sought. 
Dr. Mayer, the celebrated Austrian expert, gives the following 
formula fur a bleaching solution: 


. : 
A) Copper sulphate aio... 2) 0 eee ee ee % Oz. 20 gm. 
Waterinc: b-awa. Laois tw HE Qe eee 3% 02. 100 cc. 
B.. Potassium bromidé..):.cincutactetes a eee ¥4 OZ. 20 gm. 
Weiter oe sous was aga Maman igi eae haan 314 oz. 100 cc. 
C. Potassium: bichromiate,... cy cee Oo ee 150 “gr. — Io gm. 


Water iets sse male ee oe ae 34 oz. 100 cc. 


es on 


ars PROCESSES ~ DOL 
For use take: 
TEM iS swale halts sp bada os Dra, Cre: 60 cc. 
I OE ann ae ee O23 60 cc. 
RE AO nfo. bids, 2082 e hsb, vib o's es ov ales *% OZ. 20.CC. 
Te ny ain un & aston 4.slapieiais'n « «ne ve i See a 450 cc. 
Tepe AU | (COME... ce ees ay cess 15 drops 


Raymond E. Crowther advises the following bleaching bath which 
he claims is entirely without action on plain gelatine but exerts a power- 
ful tanning action in conjunction with the silver image: 


Copper sulphate (crystal) 10 per cent solution 170 min. 9.6 cc. 
Potassium bromide Io per cent solution........ 130. = min. ps an 
Chromic acid 1 per cent solution.............. 45 min, 27 OC, 
een es. cee cease eevee s 3% oz. 100 CC. 


This bath should bleach the image in 3 minutes at 60° F. When 
the temperature is abnormally low, say 40° F., the bath may be used 
double strength. If the print has not been completely fixed and 
washed, the bleaching operation will not be successful; it therefore 
affords a means of indicating the thoroughness of these operations.® 

Writing in the British Journal of Photography on the Bleaching of 
the Bromoil Print (1924, p. 427) H. J. P. Venn, B.Sc., strongly ad- 
vises the use of two separate baths for bleaching and tanning. Before 
bleaching the print is soaked in water for 5 minutes and then drained 
and transferred to the bleaching bath (No. 1) composed as follows: 


Copper sulphate (10 per cent eee eee er eee ere re 95 parts 
Ree GIFU Ge ici eisai = 5.5 nce wa oda a Gree 4a Me ay Moe ae Oe 5 parts 


It is then drained and transferred without rinsing to the tanning 
bath (No. 2) which consists of 


isoguer sulpiate, (10 per cent solution): i200)... ees es eae 90 parts 
Potassium bichromate {1 per cent solution) ........2..-)..00.- 10 parts 


It is allowed to remain in this bath for 4 minutes, then washed in 
several changes of water, each of five minutes’ duration, and fixed in 
a 10 per cent solution of hypo for two minutes. After about 15 
minutes’ washing the print is dried, being again soaked in water before 
inking up until the desired relief has been reached. This time will 
vary with the grade of paper. The times of soaking for a few of the 
more common grades-are as follows: 


3 A. P., 1921, p. 446; 1922, De 2 


538 PHOTOGRAPHY 


Kodak  Rovaree ion a. geciner |) ae eee 45 min. at 75° F., (ear 
Barnet: (Geter eres ooo he egy a 30 min. at 65° F. Corie" 
Wellington Bromo cco eo. isk a ee 30 min. at 65° F. Cae" 


Somewhat longer periods of soaking will do no harm but the ink 
used will then require to be slightly softer. 

To secure a print suitable for pigmenting from a contrasty negative 
increase the amount of potassium bichromate by using a 5 per cent 
solution. When used in the two-bath process this does not complicate 
the process of making. 

Chemical Theory of the Bleaching Operation.—Not a great amount 
of work has been done on the chemical theory of bromoil and little is 
known of the exact nature of the chemical reactions which form the 
basis of the process. The reactions which take place in the bleaching 
and tanning of the bromide print are, according to Mr. H. J. P. Venn, 
as follows: 


Bleaching : 
(1) Ag + CuSO, + 2KBr = Ag + CuBr + K,50,. 
Production of tanning agent: oe 


(2) 6CuBr + 6K,Cr,O, = 6CuCrO, + 6KBr + 3K,CrO, + 
Cr,O, -CrO, (chromic chromate. ) 


Tanning : 
(3) Cr,O,-CrO + gelatine = chromated or tanned gelatine.* 


The exact chemical composition of gelatine tanned by the chromates 
or the reaction between the two substances which results in the in- 
soluble condition has never been definitely ascertained although many 
authorities, notably Lumiere and Seyewetz, have done much work on 
the subject.® 

Fixing.—In the process of bleaching and tanning a secondary image 
of silver bromide is formed. ‘This image is light sensitive and, while 
not visible at the time, will appear upon exposure to light. It is there- 

4 As a result of later investigation on the theory of the bleaching bath in the 
bromoil process, H. J. P. Venn suggests the following equation as more nearly 
representing the course of the reaction in the second stage than that given 
above: 

3Cu.Cl, -- K,Cr.O, + 7H,O = 2KCl + 2CuCl, + 4Cu(OH,) + 2Cr(OH),. 


5 Lumiére and Seyewetz, Bull. Soc. franc. Phot., 1904, p. 73; 1905, p. 440; 
1905, p. 461; 1905, p. 541. 3 


ee ee ee a ee eS ee ee ee ee ee a ee 


Pe ee eee ee ee ee ee ee ee eee ee 


Ss a ee oe Orne 


ee a ee ae ae Pe ay 


THE OILePROCESSES 539 


fore necessary to fix a second time, in order to remove this silver bro- 
mide. ‘The fixing bath for this purpose consists of a plain 10 per cent 
solution of hypo. The usual thorough washing should follow the fix- 
ing operation. The print must then be allowed to dry normally.® 
Producing the Relief—When ready to begin pigmenting, the print 
is immersed in water and the gelatine allowed to swell. The degree of 
swelling is controlled principally by the temperature of the water and, 
to a lesser extent, by the time of immersion. The higher the tem- 
perature of the water in which the print is soaked, the greater the 
swelling and the more pronounced the relief. With insufficient im- 
mersion, or the use of cold water, the degree of swelling will be in- 
sufficient and such prints will, when pigmented, have a short scale of 
gradation with poor tones. The use of excessively warm water, on the 
other hand, will produce a pronounced relief which, when inked up, 
may produce a result having greater contrast than is desirable. Be- 
tween these two extremes of temperature lies an entire series of inter- 
mediate stages, which may be employed as occasion demands. 
Different papers vary as regards the temperature necessary for pro- 
ducing the best relief. Some are ready for pigmenting after soaking 
for several minutes in water at ordinary room temperature. Others 
require as high as go° F. (32° C.) or more; the general average being 
about 75-80° F. (24-27° C.). The worker must learn by experience 
the temperature to use for his particular brand of paper and manner 
of working, always remembering that it is best to start with a rather 
low degree of relief, which may be raised quite easily, 1f required, by 
soaking iri warmer water, while a relief.once too high can be reduced 
only with difficulty. Should soaking in warm water at a temperature 
of 95° F. (35° C.) be insufficient to produce the desired degree of 
relief, the print may be immersed in a I per cent solution of sodium 
carbonate as recommended by E. Guttmann. As a rule, however, this 


_ method should be used only as the last resort. 


Pigmenting.—The relief having been raised to the required stage 
the print is placed upon the wet pad of blotting paper and the surface 
moisture carefully removed with a clean, dry, lintless blotter. It is 
necessary that all the moisture on the surface be removed or the pig- 
ment will not adhere evenly. 

Pigmenting is conducted in practically the same manner as with the 

6 According to one method the prints are bleached after development and 


before fixing. This removes the necessity of the second fixing, but is not so 
reliable as the method we have described. 


540 od OD OG AOU dl on Bs 


oil process, but there are a few points which might be mentioned. It 
is best to begin with a stiff pigment in all cases and only apply the soft 
ink towards the end when it is desired to finish off the roughness of the 
gradations. Always have a margin on the original bromide print, 
otherwise there is a danger of getting the brush wet when attempting 
to pigment the edges. When the wet brush is transferred to the print 
it immediately begins to remove the pigment and the work will have 
to be done over again. 

Do not be ina great hurry. Work quickly but with care and do not 
treat the delicate gelatine surface roughly or it may be destroyed. 

Do not be afraid to apply plenty of pigment but do not try to put it 
all on at once. Smooth it down to an even tint on the palette and take 
up a little on the brush at the time. When this is exhausted take up 
more. A smooth, even tone will result if plenty of pigment is used 
and it is thoroughly worked into the surface. If the pigment is not 
well worked in, the print will be weak and “ gritty ” and the tone will 
be impure. | 

Beginners usually make the mistake of jumping about from one 
portion of the print to another. Do not do this. It only makes it 
more difficult to get an even, smooth result. Work systematically, 
starting on one side and covering the entire print as you go. 

All detail which is to appear in the finished print should be apparent 
after the first inking. If parts of the image are inked strongly before 
the desired details appear, it is difficult to ink these later. 

Particular care is necessary, especially in the case of large prints, or 
where a long time is required for inking, to keep the paper stock thor- 
oughly wet. To this end it is well to soak the print in water fre- 
quently during pigmenting. A partially dry surface is responsible for 
many of the troubles met with in pigmenting and if the print is re- 
soaked for 5—10 minutes in water whenever any difficulty is met with 
in pigmenting much better results will be secured and many of the 
supposed difficulties of the process will disappear. 

Mr. Chas. H. Partington in the American Annual of Photography 
for 1922 adopts what is probably the most satisfactory method of 
indicating to the beginner in bromoil printing precisely what brush — 
work will accomplish. By the courtesy of Mr. Partington I am able 
to reproduce the print and accompanying data, which serve to show 
graphically the effect of variations in pigmenting on the appearance of 
the image. * 

In Fig. 227, the section, A, has been pigmented with a heavily 


Seha-wies 


Pr wee Ver OCR S SES 541 


charged brush and no attempt has been made to change the result. 


This blocks the shadows and increases the gloss of the highlights pro- 
ducing a soot and whitewash effect. A better result would have been 
obtained by using less ink and a quicker action of the brush. 


Fic. 227. Results in Pigmenting. (Partington) 


At C is shown the effect obtained by not having enough ink on the 
brush. This gives a very soft, flat result. 

At D ink was applied as at 4 but “ hopping ”’ was resorted to to re- 
move the excess ink. The trees in E were “hopped” in order to 
lighten the tones and give the effect of distance. 

The portion at / shows the surface when first inked and is included 
for the purpose of showing the effect when the ink is first applied. 


36 : 


542 PHOTOGRAPHY 


At G, the print has been inked with a properly charged brush and 
slightly “ hopped ”’ to give additional contract. 

Namias Method of Pigmenting.—A very ingenious method of pig- 
menting is advised by Namias.’ 

A small portion of hard ink is carefully mixed with ten times its 
weight of finely rectified turpentine. The brush is at once charged 
with this mxiture and the operation of pigmenting begun as usual. 
The surface of the print quickly becomes covered all over with a fine 
coating of ink. Continue the dabbing action of the brush. Gradually 
the shadows appear to take on ink and gain intensity, while the details 
and half-tones become clearly separated from the highlights. As the 
turpentine evaporates, both on the print and on the brush, the ink 
becomes thicker and thicker and as it increases in consistency it begins 
to pass the print to the brush in the highlights, and from the brush to 
the print in the shadows. Thus in a very short time the image be- 
comes built up to a surprising extent. 

The first part of pigmenting complete, a thicker ink is used. Three 
parts of hard ink and two parts of soft ink are dissolved in 4-5 times 
its weight of 


Rectified ‘turpentinék.. salsa ee Jn sce Oe aes ‘-%, part 
Gasoline (pure)... \.c0 cea ys wea wan te Pace «9c 2 parts 


This second ink is applied in much the same way as the first but is 


intended for the lighter tones and must be used considerably harder 


than the first, or the former will be removed. | 

Should the final result be unsatisfactory the print may be swabbed 
with absorbent cotton saturated with benzol. This will remove every 
trace of the pigment. : 

The treatment of the finished bromoil after pigmenting is identical 
with the oil print already described. 

Defatting the Firtished Bromoil.—After the print is dry it is well 
to remove the oil which is included in the ink, and which has the effect 
of giving a slight gloss to the print. The sheen is greater in the 
shadows than in the highlights especially if a soft ink has-been used, 
as soft inks contain a larger percentage of oil. To many this gloss 
constitutes an objection, while there is in addition the danger that the 
ink may in course of time, through oxidation, give rise to colored 
stains. 

For removing this oil some solvent such a benzol, carbon tetrachlo- 


7 Brit. J. Phot., 1914, 61, 626. 


nt 


~ A iS Tes - = 


THEO PROCESSES | 543 


ride, etc., should be used. Owing to the fact that soft ink may be dis- 
solved by carbon tetrachloride, which is a more energetic solvent than 
benzol, the latter is preferable. It is poured in a dish and the per- 
fectly dry print immersed in the liquid for 5-10 minutes. 

Bromoil Transfer.—Oil transfer, first introduced by Robert Dem- 
achy about 1906, is now one of the most popular printing mediums 
among advanced pictorialists. Bromoil transfer, a natural develop- 
ment of oil transfer, consists, as its name indicates, in transferring the 
pigment from the original bromoil to a-sheet of plain paper which may 
be of almost any surface, texture or color. As the greasy pigment on 
the bromoil lies on top of a more or less tanned and swollen gelatine 
film, when brought into contact with any uncoated paper and passed 
between rolls under pressure, it will leave the bromoil print and adhere 
to the plain paper. The image in this case, then, consists of pigment 
on a plain paper base. ‘Transfers, accordingly, have a distinctive ap- 
pearance entirely unlike that of any other printing process, with the 
exception of photogravure, since in all other processes the image is 
imbedded in gelatine or in some other colloid, while in these two proc- 
esses the image lies on a plain, uncoated paper. Added to this is the 
advantage of being able, by combination transfer, to extend the scale 
of gradation, and exercise over the finished result a degree of control 
which is beyond the limits of even the bromoil process, as flexible as 
this may be in the hands of the expert. 

Making a really good transfer is not as simple as might be assumed 
from an outline of the operation. Familiarity with the bromoil proc- 
ess, even, does not assure the worker of being able to produce ac- 
ceptable transfers at the start; only by constant experiment and study 
can one hope to master the process. But the results are such as to 
amply repay one for the labor involved in mastering the process and 
one who has become thoroughly familiar with oil or bromoil should not 
rest satisfied until he has also attempted transfer. 

The Bromide Print.—In general bromide papers which are suit- 
able for bromoil are also adapted to bromoil transfer. According to 
Mr. C. J. Symes, super-coated bromide papers (i.e. papers which have 
received a double coating in order to render them non-abrasive) yield 
particularly fine transfers under certain conditions, namely : 

(1) The image must be rather stronger than for bromoil; there 
must be a distinct veiling of the highlights and the print, as a whole, 
must be a shade darker than if the straight print were intended for 
exhibition. : 


544 PHOTOGRAPHY 


(2) Development of the bromide print must be full. If the kodak 
amidol formula is used the print must be developed to a Watkins fac- 
tor of at least 16. 

(3) The print must be swabbed with cotton wae before inking, if 
a bleacher of copper chloride, hydrochloric acid and bichromate is 
used. | 

(4) Each batch of paper must be tested for the time of soaking, 
owing to possible variations in the super-coat.® 

Preparation of the Bromoil.—To obtain a transfer of peda aaatiey 
soft ink must be used in pigmenting the bromoil as it is impossible to 
transfer hard ink with certainty, owing to the tenacity with which it 
adheres to the original bromoil. Soft ink, however, cannot be used 
unless a high relief is obtained or the ink will adhere to the highlights 
of the bromoil and a print of the proper gradation cannot be obtained. 
Consequently it is necessary to start with a rather high relief; this fact 
must be borne in mind when the print is being made ready for pig- 
menting and the temperature of the water in which the swelling takes 
place regulated accordingly. The use of a high temperature, however, 
may cause the gelatine in the highlights to soften to such an extent 
that it pulls off in pigmenting or in transfer. When this occurs it 
is well to make use of ammonia as previously described. As the 
ink is more easily transferred from the highlights than from the | 
shadows, in consequence of the greater relief of the former and the 
fact that owing to the tanning of the* gelatine the pigment in the 
shadows is more strongly retained than in the highlights, the contrast 
of the transfer is usually much less than that of the bromoil. In pig- 
menting, therefore, the bromoil is made considerably more contrasty — 
than would be required were it to be left as it is. Owing to the fact 
that the transfer of ink in the shadows may not be complete, it is the 
practice of many workers to considerably over ink such portions in 
order that the transfer may have the proper depth in the shadows. © 
To reach the same end other workers have recourse to multiple trans- 
fer; the first bromoil. being inked normally and transferred, then 
reinked, paying especial attention to the shadows. 

Dr. Emil Mayer has found that the difficulty of transferring a 
bromoil to transfer paper without loss of depth in the shadows, due 
to an incomplete transfer of ink, may be overcome by first passing the 
transfer through the press with comparatively light pressure, then 


8 Brit. J. Phot., 1923, 70, 103. 


THE Oll. PROCESSES 545 


separating the bromoil and the transfer paper (without shifting their 
relative position) so as to expose both surfaces to air. Then place the 
two in contact and run through the press the second time with in- 
creased pressure. With this procedure the transfer of ink is almost 
complete and there is no necessity for over pigmenting of the shadows, 
or for a second inking. He also finds that there is no advantage in 
passing the transfer through the press repeatedly with increased pres- 
sure, when this procedure is followed, as the transfer of ink takes 
place immediately and increased pressure only serves to produce an un- 
necessary strain on the gelatine.° 

The Transfer Paper.—Theoretically any paper should be suitable 
for the transfer but in practice there are some marked limitations. 
Without going into a detailed discussion of the adaptability of various 
makes of papers, it may be said that only pure rag paper is suitable. 
The commercial water-color and drawing papers of reliable makers 
are, as a rule, suitable for the transfer, as are the Japanese and 
Chinese papers, but the very best paper is that manufactured especially 
for copper-plate printing. In general, however, the worker will not 
have much difficulty in using reliable makes of drawing papers, such 
as, for example, the Strathmore papers of the Mittineague Paper Co. 
which are obtainable from most dealers in art goods. 

With very absorbent papers sizing may be necessary, as the pig- 
ment sinks into the pores of the paper and the picture has a flat, 
“sunken in” appearance. For this purpose make up the following 
solutions : 


eee eR hea cs ces oh dees eadyes- LOO CC 2 OZ: 


The arrowroot should be rubbed up with a small quantity of water 
and added with constant stirring to sufficient boiling water to make a 
total volume of approximately 100 cc. (3 0z.). This is applied with 
a Blanchard brush or tuft of absorbent cotton and the paper allowed 
to dry without heat when it is ready for use. 

As a general rule the transfer paper should be used dry. There are 
some few papers, however, which require to be slightly dampened. 
For this purpose it is sufficient to thoroughly and evenly dampen two 
sheets of blotting paper and place the sheet of transfer paper between 
them and under slight pressure for several minutes. As different 


® Amer. Phot., 1924 (July), p. 410; Brit. J. Phot., 1924, 71, 412. 


546 PHOTOGRAPES 


2 

i 
papers act differently in transferring, the beginner should stick to ene 
make and surface of paper until he is thoroughly familiar with it. — 
Then he may, if he desires, experiment with other makes and surfaces. | 
The Transfer Press.—Special presses for transfer are supplied by _ 
the Autotype Co. of London and by Sinclair, also of London. These . 
i 


Fic. 228. Transfer Presses 


are similar to the presses used by copper plate printers and are ex- 
pensive. Very good work can be done, however, with one of the 
old burnishers as used in past days for the glazing of prints or with 
the better grades of domestic wringers. Whatever the type of press 
it should satisfactorily fulfill two requirements: (1) the pressure on 
the rolls must be absolutely even and capable of regulation by the 
worker and (2) one must be able to examine the condition of the 
transfer at any time without danger of shifting the position of the 
bromoil or the transfer. 
We illustrate in Fig. 228 the Autotype and Sinclair presses. 
Perhaps one of the best forms of press for bromoil transfer is that 


THE OIL PROCESSES 547 


described by K. Prett.*° This press, which is shown in Fig. 229, is 
similar to that used by collotype printers. The bromoil in contact with 
its transfer paper is placed between the two plates of the pressure pad, 
P. ‘The pressure is then adjusted as required by means of the pres- 
_ sure bar, C, and the movable pressure pad, P, is drawn underneath the 
pressure bar, C, by the windlass, W. The whole affair may be made 


Fic. 229. Prett’s Transfer Press 


of wood; the two plates composing the movable pressure plate, P, 
being lubricated with a little talc to make them slide regularly. The 
pressure which can be attained exceeds that of the roller type of press 
while there is no danger whatever of a displacement of the bromoil 
_and there is less wear on the original bromoil than possible with any 
other type of press. 

Transferring the Pigment.—As soon as pigmenting is complete the 
bromoil is ready for the transfer. For this purpose we require, in 
addition to a suitable press, two sheets of blotting paper, three sheets 
of thick, hard, glazed pasteboard and a pad of felt—all of which 
should be at least double the length of the bromoil print. On one of 
the sheets of pasteboard is placed one of the sheets of blotting paper 
and on this the pigmented bromoil, face up. Over this is placed the 
transfer paper, and over this another sheet of blotting paper. These 
two sheets of blotting paper serve the purpose of absorbing the mois- 
ture squeezed from the bromoil print which might otherwise cause 
trouble. Finally a sheet of pasteboard is placed over the blotting 
paper, then over this the felt pad and lastly another sheet of paste- 
board. 


10 Phot. Rund., 1923, 1,5; B. J. P., 1923, 70, 300. 


ae © ee 


548 PHOTOGRAPHY 


The entire pack is now inserted between the rollers and carried 
through once with a uniform motion and with but slight pressure. , 
The pressure is then increased slightly and the pack carried back 
through the press in the opposite direction. Then the top of the press 
pack is removed, the cover of the transfer paper raised and the ap- 
pearance of the transfer examined. If the transfer of ink is only 
slight, the press pack is replaced and carried through the press again 
with increased pressure. Then if the shadows still lack intensity reg- 
istration marks should be made, the bromoil print removed, resoaked 
in water and the shadows pigmented after which the bromoil is placed 
on the transfer paper, its position registered, and again passed through 
the press. The pressure should not in any case be so great that the 
rolls can be started only by a decided effort; they must always 
move easily and smoothly. -“ Repeated slow passage of the press 
pack through moderately tightened rollers is always more advanta-. 
geous than a single passage under very heavy pressure.” ** With 
heavy pressure there is likewise the danger of destroying the bromoil, 
as the gelatine film in its swollen condition may adhere to the trans- 
fer paper. This trouble, however, is occasionally met with when 
using some papers with only a moderate amount of pressure. To 
prevent this, Dr. Mayer suggests that the transfer paper be sprayed 
with oil of turpentine by means of an atomizer.” 

After spraying the sheet is allowed to stand for fifteen or twenty 
minutes in order that the turpentine may evaporate. ‘This is a cer- 
tain preventative of sticking, but sufficient time must be allowed for 
the turpentine to evaporate, or muddy, uneven transfers will result. — 

Zaepernick’s Chemical Transfer Method.—In American Photog- 
raphy, 1924, p. 732, Hans Zaepernick describes a method of bromoil 
transfer which he terms chemical transfer. He says: 

The chemical transfer in its simplest form consists of dampening the paper 
on to which the bromoil, prepared in the usual way, is to be transferred, not with 
water, but with a solvent of the ink. The solvents for the greasy inks are: 
petroleum ether, benzine, benzol, and oil of turpentine. The transfer of the ink 
from the bromoil to the new surface is effected after solution has taken place 
by the absorption and adhesive power of the transfer paper. For perfect trans- 
fer of the ink, light pressing together of the two surfaces is essential. Even 
the light pressure obtainable in a printing frame or light rolling with a roller 
squeegee is enough. 

If this method of working is adopted, the bromoil should only be lightly inked. 


11 Guttman, Bromoil Printing and Transfer, p. 164. 
12 Brit. J. Phot., 1924, 71, 412; Amer. Phot., 1924 (July), p. 410. 


THE OIL PROCESSES 549 


If the inking has been too heavy, the transfer will, as a rule, be too plucky as all 
the ink goes on to the transfer paper. The degree of hardness or consistency 
plays but a subordinate part in this process. 

If oil of turpentine is used for dampening the paper, black inks show a brown- 
ish tinge. With benzine this does not. occur. 

The advantages of chemical transfer are that since but little pressure is re- 
quired, it is not necessary to invest in an expensive press and that any kind of 
paper, even the extremely thin Japanese tissue, may be used. 


Rowatt’s Process.—In the Club Photographer for February 1922, 
157, Mr. J. Rowatt describes a method of offset bromoil transfer 
which he claims removes most of the difficulties of the ordinary 
bromoil transfer. The pigmented image of the bromoil is transferred 
to a rubber blanket and from the latter to the final support. The 
bromoil print accordingly does not require to be reversed as in ordi- 
nary bromoil transfer, when unreversed prints are required. For 
more complete details we must refer the reader to the original. 

Multiple Transfer.—Multiple transfer is employed in the same gen- 
eral way and for the same purpose as in gum-bichromate printing— 
namely, to lengthen the scale of gradation in order that every pos- 
sible tonal value contained in the negative may be properly rendered. 
The multiple transfer may be made from one or more bromoils. If 
only one bromoil print is used, it is first inked up with hard ink, so 
adjusted to the relief of the print that the shadows alone absorb any 
considerable quantity of ink, the lighter half-tones and highlights re- 
maining untouched. This corresponds to the shadow coating in gum- 
bichromate. The pigmented image having been transferred to the 
transfer paper and means of registration provided in order that it 
may be placed again in identically the same position, the bromoil is 
again pigmented, but this time with a soft ink so as to produce a thin, 
smooth film of ink which reproduces the highlights and half-tones 
while adding but little, or not at all, to the shadows. This transfer 
obviously corresponds to the highlight coating in gum-bichrornate. 

Instead of using the same bromoil print for both transfers, two 
separate bromoils may be used. This method has the added advantage 
that different papers may be used for the two bromoil prints, and that 
the degree of relief of the two prints may be regulated so as to more 
easily obtain the effect desired in pigmenting. 


550 PHOTOGRAPHY 


GENERAL REFERENCE WorRKS 


DEMACHY AND Puyo—Les Procedes D’Art en Photographie. 

Duvivier—Le Procede a L’Huile en Photographie. 

Eper—Das Pigmentverfahren, der Gummi-, Oel-, und Bromol-druck und ver- 
wandte photographische Kopierfahren mit Chromalzen. 

FuHRMANN—Der Oeldruck. 

GutrMAN—Die Se‘bstbereitung der Bromoldruckfarben. 

GutrMAN—Der Umdruck in Bromoldruckverfahren. 

KuHN—Technik der Lichtbilderei. 

LAMBERT—Oil and Bromoil. 

Mayer—Das Bromoldruckverfahren. 

Mayer—Bromoil Printing and Transfer. English translation by Fraprie. 

Meses—Der Bromoldruck. 

Puyo—Die Oilfarben-kopierprozess. 

Puyo—Les Procedes aux encres Grasses. 

SINCLAIR—How to make Oil and Bromoil Prints. 

TILNEY AND Cox—The Art of Pigmenting. 

TILNEY AND JupGE—Oil and Bromoil Transfer. 

MortTIMER AND CouLTHuURST—The Oil and Bromoil Processes. . 

STENGER—Neuzeitliche photographische Kopierverfahren. (Ozobrom, Brom- 
silber, Pigmentpaper, Oldruck, Bromoldruck.) 

STENGER—Die Kopierverfahren, 1926. 

Photo-Miniature No. 106—The Oil and Bromoil Processes. 

Photo-Miniature No. 186—Bromoil Prints and Transfers. 


, 
: 
J 


CHAPTER AXV 


COPYING 


Introduction.—Copying is a branch of photography in which many 
do not succeed, not because of any inherent difficulties the work pre- 
sents, but because the essentials of the subject which are necessary to 
success are not thoroughly understood. With the proper apparatus 
and materials and an understanding of the factors involved, copying is 
in no ways more difficult than other photographic work and provided 
the worker knows what he is about he should meet with but little diffi- 
culty. 

In discussing the subject we will consider first the apparatus ad- 
visable, then the optical principles involved and the proper treatment 
for different classes of copies and finally the photographing of small 
objects in the studio. 

Apparatus for Copying.—In hardly any branch of ordinary photo- 
graphic work is apparatus so important as in copying and for this 
reason the question of equipment should be settled before the work 
is begun. In the first place it is essential that some means be pro- 
vided whereby the camera may be moved to or from the subject with- 


=— 


2s 


—— 


= —_ 
- — 


Fic. 230. Copying Stand Fic. 231. Book Holder for Copying 


out destroying the parallelism necessary to prevent distortion. Stands 
for this purpose are made by several firms or.a simple arrangement 
may be made at home by anyone familiar with tools. Figure 230 shows 
a simple fixture which fills all ordinary requirements and is of simple — 

551 


002 3 PHOTOGRAPHY 


construction. The essential parts are the tracks AA on which the 
camera moves to and from the easel C which is rigidly fixed at right 
angles to the base B provided for the camera. In cases where an 
apparatus like this cannot be used, as when photographing a large map 
or oil painting, a small celluloid T square should be used to determine 
if the image of the subject on the ground-glass is free from distortion. 
For copying from books a holder such as illustrated in Fig. 231 is 
very convenient. On the whole, however, it is much simpler to use a 
vertical stand, as it is much easier to keep the page flat when the book 
is in this position. In fact, a-vertical stand is more convenient for 
nearly all general copying as there is no trouble in attaching the print 
to the easel and, if daylight be used for illumination, it is easier to 
secure uniform illumination with the print in this position. Many of 
the stands on the market may be used vertically as well as horizontally 
and, as we will see later, the possibility of using the stand in a vertical 
position is particularly advantageous in another form of copying. 
Methods of Illuminating the Print.—The light which illuminates 
the print to be copied should not only be evenly distributed over the 
whole print but it should also come from more than one source. The 
reason for this will be all the more apparent when we have to deal with 
papers of coarse and irregular texture such as used for drawing pur- 
poses. A side lighting from a single concentrated source accentuates 
the graininess of surface by causing the innumerable projections to 
cast shadows on the side away from the light. At the same time the 


projections themselves receive the direct illumination on one side and © 


therefore we have a highly lighted spot in immediate contact with a 
deep shadow so that the irregularity in the surface of the paper is made 
far more noticeable than is actually the case and the copy shows a 
“ graininess ’ which is almost inconceivable when the original is ex- 
amined visually in a good light. 

When copying by daylight it is very difficult to secure uniform 
illumination and prevent the appearance of an undesirable amount of 
“eraininess.” All papers which do not have a glazed surface may be 
copied in the position shown in a of Fig, 232. For glazed prints this 
position is unsuitable, as the highly glazed surface reflects light into 
the camera and obscures the image. In such cases, and also in the 
case of some matt papers which have a “ velvet ” or enamelled surface, 
the relation between the print and the light source should be that shown 
in b of the same figure. The presence of reflections can usually be 
determined from the ground-glass but an infallible rule is to remove 


——— 


COPYING 553 


the ground-glass and the lens and examine the print from the back of 
the camera at various angles. 

The constant fluctuation in the strength of daylight and the difficulty 
of securing even illumination make artificial illumination especially 
desirable. For several years the writer used with complete success 
the arrangement illustrated in Fig. 233. The interior of the box which 


Fic. 232. Illumination of the Copy Using Daylight 


encloses the mazda lamps is painted with white enamel to increase the 
reflecting power and the bulbs are placed back from the circular open- 
ing so that no stray light can reach the camera even when very close 
to the copy. Frosted light bulbs were found to give better illumina- 
tion with less tendency to reflection and glare than plain bulbs. While 


BiG. 29%: Copying Apparatus for Artificial Light 
(Rose, The Commercial Photographer) 


somewhat elaborate this outfit is easily constructed and fully repays 
its expense where a considerable amount of copying must be done, for 
the circular system of lighting is the most effectual way of avoiding 


_“ graininess” that the writer has been able to find. Two mercury 


vapor tubes, one on each side of the copy, make a satisfactory light 
but the initial expense is higher. Two large mazda lights, one on each 
side of the copy, are sufficient when dealing with small copies but fail 
with very large originals and it is also difficult at times to avoid reflec- 
tions. . 


554 PHOTOGRAPHY 


Copying Cameras.—For copying the worker has the choice of the 
instruments made especially for the purpose or the use of view or 
other types of plate cameras. Regular copying cameras have a long 
bellows, a central compartment for lenses and quite often a frame of 
kits at one end for negatives which are to be reduced to lantern slides. 
When necessary the lens board may be moved from the central par- 
tition, where it is placed when making lantern slides, and substituted 
for the frame of kits in order to obtain greater bellows capacity. 
Cameras of this type are made by a number of firms and reference to 
the catalogs of large dealers will show what may be expected in a 
camera of this type. 

Long bellows view cameras which focus from the rear are very 
satisfactory for copying: in fact in one respect they are actually more 
convenient than those made especially for the purpose. Copying 
cameras are seldom fitted with a rising and falling front, but this 
feature is very convenient at times as it enables the image to be prop- 
erly adjusted on the ground-glass without the rather laborious operation 
of removing the print from the easel and replacing it in what is esti- 
mated to be the correct position. The swing back fitted to view 
cameras is also useful at times, enabling distortion in the copy to be 
corrected. At all other times it should be securely locked in the 
perpendicular position so that it is parallel with the easel. 

Copying with hand cameras having short bellows is possible only 
when supplementary lenses are used. These, while useful in such 
cases, cannot be recommended as they affect the definition of the ob- 
jective to which they are applied. 

The Objective for Copying.—Difficult copying demands high-grade 
objectives. While very good copies can be made with rapid rectilinear 
lenses these have a falling off in definition towards the margins in 
addition to astigmatism, both of which are only partially remedied by 
stopping down. ‘The anastigmat with its flat field and its high free- 
dom from all kinds of aberration gives critical definition and needs 
but little stopping down, so that for all work which demands utmost 
sharpness they are far superior to other types of lenses. Lenses well 
corrected for astigmatism and curvature of field may still have zonal 
errors or residual spherical aberration. Zonal aberrations detract 
from the crispness of definition and reduce the limit of sharpness to 
which one can work. Coma is also a serious disadvantage to a lens 
used for copying as it gives negatives having a flat, fogged appearance 
which is sometimes mistaken for errors in exposure or development. 


COPYING 555 


Lenses of medium aperture are superior in these respects to those of 


_large aperture, even though both be used at the same aperture, owing 


to superior correction for zonal aberration. Unquestionably the best 
lens for all classes of copying is the process anastigmat, such as the 
Cooke Series V, //8, Goerz Gotar, F/8, Gundlach process F/9, Velo- 
stigmat process /’/8, etc., but anastigmats of the type represented by 
the Dagor, Protar, Turner-Reich, and Tessar 11B, /’/6.3, are satis- 
factory for all but the most critical line work. 

While the use of a short focus lens means a saving of bellows ex- 
tension and allows the camera to be closer. to the copy for a given 
degree of reduction or enlargement, it has the disadvantage that the 
front of the camera may, in certain cases, interfere with the lighting 
of the subject while at the same time the danger from reflections is 
greater owing to the larger angle subtended. In general it is well to 


-choose a lens having a focal length equal to, or slightly greater than, 


the diagonal of the largest plate. 

Focusing.—For accurate focusing a fine-grained screen is needed. 
Much may be done to improve matters by simply applying vaseline to 
the ground-glass already in the camera but a much better result can be 
secured by replacing the ground-glass with a specially made screen. 
A very suitable grainless screen can be made at home at a very small 
expense. Take a fast plate (unexposed) and develop from fifteen to 
twenty minutes in a non-staining developer such as amidol or M—O 
without a restrainer so as to secure a slight general fog. Rinse and 
transfer to the following solution: 


PSE 0 IO gr. 25 gm. 
ee AiG AE CONC. ke es ee ee ieee es 10 min. eS CC, 
RR ee, Vy cy ey ne Sine cime ne ctos I Oz. 1000 cc. 


After several minutes’ immersion in this, remove and rinse briefly in 
running water, then fix, wash and dry in the ordinary way. A screen 
prepared in this manner is denser than one of ordinary ground-glass 
but shows far more detail owing to its freedom from coarse grain. 
When dry it is well to rule the screen with vertical and horizontal 
lines 14 inch apart to assist in determining the size of the copy directly 
without measurement and to indicate the presence of distortion. When 
this is done, the screen may be coated with negative varnish to protect 
it from atmospheric action. 

For obtaining critical focus a PAnenineE must be used. The parallax 
focusing method, or the use of the Le Clerc diaphragm, in conjunc- 


556 PHOTOGRAPHY 


tion with a focusing magnifier affords the simplest and most satisfac- 
tory method of obtaining the exact focus. 

To use the former method proceed as follows: 

Remove the gelatine coating of the prepared focusing screen from 
a small portion about an inch in diameter at the center of the screen. 
On this clear space glue a piece of tinfoil with a sharp edge. A 
magnifier is adjusted to sharp focus over the tinfoil and may be 
permanently affixed in this position. As the eye moves sideways in 
observing the image an apparent displacement occurs. When critical 
focus is secured there is no apparent displacement and the image and 
the sharp edge of tinfoil lie in the same plane. 

Clerc’s method may be used only when the lens is fitted with re- 
movable diaphragms, generally termed Waterhouse stops. As practi- 
cally all process anastigmats are fitted with removable diaphragms this 
method becomes very convenient when such lenses are used. ‘To pro-. 
duce the Clerc focusing diaphragm lay off on thin metal a circle equal 
to the diameter of the inside of your lens barrel. Inside of this circle, 
lay off a concentric circle equal to the diameter of the largest diaphragm 
of the lens. Draw a diameter of the inside circle and divide into four 
equal parts, and at the two points between the center and the circum- 
ference of the circle draw perpendiculars to the diaphragm until they 
cut the circumference of the inner circle. Then cut out the segments 
and blacken the metal with dead black, matt paint. When focusing 
with the diaphragm in place there will be a double image but when 
critical focus is obtained the images unite and form a single distinct 
image. Remove, insert proper stop, and expose. 

Copying to Scale—Assuming that the exact focal length and the 
position of the nodal points are known, the worker can enlarge or 
reduce to scale simply by a graduated scale applied to the camera and 
the stand. The conjugate distances for various degrees of enlarging 
or reducing and for lenses the focal length of which varies from 3 to 
12 inches are given in the following table. When copying on an en- 
larged scale the distance from the subject to the lens is less than that 
from the lens to the plate while when copying on a reduced scale the 
reverse is the case. 

Where the positions of the nodal planes are unknown, the following 
method worked out by Mr. D. Charles? may be employed: 

The first requirement is that the ground-glass focusing screen 
should allow of horizontal movement in its frame over a small distance 


1 Brit. J. Phot., 1919, 66, 736. 


COPYING 557 


DISTANCES WHEN ENLARGING AND REDUCING 


Times of Enlargement and Reduction 


Focus of 
Lensg,. . 
Inches I Inch 2 Inches | 3 Inches | 4 Inches | 5 Inches | 6 Inches | 7 Inches | 8 Inches 
<%, 6 9 12 15 18 21 4 27 
6 4s 4 3/4 3°/s 31/2 3°/r 3*/s 
3/2 7 10!/s 14 17'/, | 21 24/2 | 28 31/2 
7 51/1 4?/s 4°/s 41/5 44ie 4 3 /i6 
4 8 12 16 20 24 28 3 36 
8 6 5}/s 5 44/5 4?/3 44/7 4*/2 
41/2 9 134, | 18 22\/ | 27 30/2 | 36 40'/s 
9 63/4 6 5°/s 57/5 5'/4 51/1 51/16 
5 10 15 20 25 30 a 
10 7'/s 67/3 61/4 5°/6 5°/1 5°/s 
51/2 II 16'/2 | 22 27'l2 | 33 38'/ 44 49'/2 
II 81/4 7/s 67/s 63/5 65/19 62/7 63/16 
6 12 18 24 30 36 42 48 
12 9 8 7/2 71/5 7 68/1 63/4 
7 14 21 28 35 42 49 56 63 
14 10/5 9/3 83/4 8?/; 81/6 8 7'Is 
8 16 24 32 40 48 56 64 72 
16 12 102/s 10 93/5 9!/s 91/7 9 
9 18 2 36 45 4 63 7a 81 
18 13!/o 12 11!/4 104/; 10!/ 102/7 10!/s 
10 20 30 40 50 60 70 80 
20 I5 13!/3 I21/p 12 112/3 114/ Tafa 
II 22 33 44 55 66 77 88 - 99 
7 16!/» 14?/3 13/4 1345 125/¢ 124/7 123/s 
12 24 36 48 60 no 84 96 108 
24 18 16 15 14?/5 14 13°/7 13}'/2 


The table is used as follows: Knowing the focal length of the lens to be 
used and the degree of (linear) enlargement or reduction, look up the figure 
for enlargement or reduction in the upper horizontal row, and carry the eye 
down the column below it until it reaches the horizontal line of figures op- 
posite the focal length of lens in the left-hand column. 

When enlarging, the greater of the two distances where the two lines join is 
the distance from lens to the sensitive paper or plate. The lesser is the dis- 
tance from lens to negative, or picture being enlarged direct in camera. 

When reducing, the distances are vice-versa: the greater is the distance from 
lens to original, the smaller from lens to sensitive plate. (British Journal of 
Photography.) 

37 


558 PHOTOGRAPHY 


of 14 to % inch, as may easily be done by cutting a strip of this width 
off one end of the focusing screen. 

A pair of fine lines is then drawn on the focusing screen exactly 
vertical and two inches apart. The left line should be in the center 
of the ground-glass and the other two inches to the right of it. 

On the copying easel an accurately graduated scale is fixed: a paper 
scale may be glued to the surface of the easel or a wooden or metal 
scale set in flush with the surface. A vertical line is drawn on the 
easel close to the center, so that its image will coincide with the central 
line on the focusing screen. ‘The scale should be fixed about halfway 
up the easel at right angles to the central line, with its zero on the line 
and the graduations lying to the left and upside down. 

It is a very simple and rapid operation to slide the ground-glass so 
that the left hand falls on the zero of the image scale and to note the 
figure cut by the right-hand line. Thus it is possible to measure in- 
stantly the image of the rule by the two-inch column on the ground- 
glass and by focusing and movement of the camera get any desired 
degree of reduction or enlargement. 

Where it is required to copy subjects to exact size, or to a certain 
degree of reduction, at frequent intervals it is convenient to mark on 
the camera and the stand the positions occupied so that focusing may 
be avoided in the future. 

Exposures in Copying.—Five things determine the time of ex- 
posure in copying: 


. The strength of the light illuminating the copy. 

. The character of the original to be copied. 

. The speed of the plate used. 

. The actual aperture of the lens. 

. The effective aperture of the lens for the degree of reduction or en- 
largement being made. 


wm BW NN & 


The strength of the light illuminating the copy is constant when . 


artificial light is used and may be determined with sufficient accuracy 
for all practical purposes by a few trial exposures. When copying by 
daylight an actinometer should be used. 

The second factor is the one giving the most trouble since it follows 
no definite law and does not permit of measurement conveniently. 
Only experience can show what allowances must be made for different 
types of originals although the following table may be of some as- 
sistance in this respect. 


a a ee 


COPYING 559 


Fraction of 
the total Wat- 
kins meter 
Original Relative time time 


Matt or setui-matt bromide prints, platinums, pencil 


or ink sketches, steel or wood engraving........ : COMES, PR! Cire ae aare 14 
Glossy purple P-O-P contrasty bromide prints, black 

carbon, black photogravure...... ab aie SRE TS 3 TU hh Saath eae Ve 
Eichings in brown, sepia-toned bromides, red or green 

SO ee RAs a re 11 Bo ae ae ee ee ae 1% 
Contrasty sepia and red prints, gum bromoil and 

ee iv ah wclse Gs le a ae a ae ae A 


The type of plate used depends to a certain extent upon the class 
of subject: thus for line work in pure black and white a process plate 
must be used; for colored subjects an orthochromatic or panchro- 
matic plate is required while medium speed, non-color-sensitive plates 
are satisfactory for photographs and like subjects in monochrome. 
This matter will be discussed more fully when we come to deal with 
the handling of these various classes of subjects. 

With a suitable lens there is no necessity for the use of a very small 
diaphragm provided focusing has been properly done. Larger aper- 
tures tend to produce more brilliant negatives and lessen the danger 
of unsharpness due to vibration during exposure. If the lens is at all 
suited to the purpose there should be no need whatsoever for the use 
of a smaller diaphragm than F'/16. 

The values of the various diaphragms, however, are not constant 
as in general work but vary considerably with the degree of reduction 
or enlargement. Thus when copying full size the distance from the 
nodal plane to the plate is twice as great as the focal length of the 


_ lens: hence under these conditions the actual value of the stop has in- 


creased four times so that f/8 has become F/16, F/11.3 has become 
F’/22, etc. When copying on an enlarged scale the increase is much 
greater. By working with a certain definite diaphragm the relative 
exposure for copying or reducing may be calculated from the follow- 
ing table provided the correct exposure for the same class of subject 
and the same plate is known for a given degree of reduction. 

Provided it is possible to illuminate the copying easel with lamps 
of sufficient brilliance to enable them to be retained at a fixed distance 
for all originals regardless of size, exposures in copying may be cal- 
culated quite simply by the method described by Mr. D. Charles in 
the British Journal of Photography.’ 


2 Brit. J. Phot., 1922, 69, 709. 


560 PHOTOGRAP ES 


RELATIVE EXPOSURES WHEN COPYING OR REDUCING 


New Scale of Reduction for which Exposure is Known 
Scales 
of 
Reduction I 3/4 2/3 1/o 1/g 1/4 1/5 1/6 L/g 1/10 1 /o9 1/39 
I I I/, | to | 18/4 | 24a | 242 | 3.1 Be SIS inane eee 
3/4 3/4 I 11/10 13/4 13/4 2 2 21/4 2!/» 21/2 3 
*/s Ye} He |r | Uf | fe | i | 2 | 2 ae ee ee 
1/y 3/5 3/4 4/s I 1}/, 11/9 1}/, 13/, 2 2 2 2 
1/5 ah 3/5 2/5 gia Ye | U/s | B/s | fe | Je | fe | 15/4 
1/4 gL meek ee oe ge NP I Me | U/s | U/s | Uf2 | U/e 
‘/s Ss | Me | Mattei leat I fal) | Ti/eaaVe arrears 
1/¢ eRe) Af ale NP aaa I I Ye | U/s | 1/4 
Ys Me | "ls | "es "Js | "se | Sees I I I/io| /s 
Io Ys) 2s}. Aled Me) a) Shee I I 1'/g 
log an hed od sa FP oe 2/5 ah, eh ad I I { 


To use this table find in the top horizontal line the scale of reduction for which 
exposure is known. Under this scale the relative time of exposure for different 
degrees of reduction will be found opposite the new scales of reduction marked in 
first vertical column. 


The power and position of the lamps having been standardized at 
the start, a lens is placed on the camera and the diaphragm adjusted 
so as to be exactly one inch in diameter. This for an eight-inch lens 
would be F/8, fora lens of eleven inches focal length F/11, ete. The 
camera is then extended so that the distance from the stop to the plate 
is sixteen inches and the whole camera moved back and forth until 
some matter on the easel becomes critically sharp. A print is then 
pinned up and a plate exposed in steps, developed and the correct ex- 
posure noted. 

A sheet of paper is then inscribed with the ordinary apertures from 


7 
4 
F 
4 


F/6 to F/45 inacolumn. Opposite F/16 is written the exposure ar- - 


rived at by the test just described. (It will be evident upon considera- 
tion of the conditions under which the test was made that since the 
lens opening of one inch is one sixteenth of the bellows extension, the 
effective aperture, or the actual working speed, of the lens is F/16 


regardless of what the aperture marked on the lens mount may be.) 


Opposite each other diaphragm is written the proportionate exposure 
following the usual rule. 


COPYING 561 


This procedure is then repeated for a line subject using a process 
plate.and for any other particular class of work which requires dis- 
tinct treatment, and the corresponding exposures marked against each 
diaphragm. 

A scale is then affixed to the camera so that zero point coincides 
with the diaphragm of the lens. Obviously, the actual extension of 
the camera can be determined by observing the figure against which 
the ground-glass stands. From what has been said before it will now 
be evident that the extension indicates the value of the stop without 
any calculation whatsoever. ‘Thus if the extension is eight inches the 
exposure would be read off opposite F/8, if 22 inches opposite F/22, 
etc. If for any reason it is necessary to use a smaller diaphragm the 
proper exposure may be determined by the usual rules governing the 
exposures of different diaphragms. 

The Copying of Subjects in Pure Black and White.—Having be- 
come familiar with the fundamental principles underlying all copying 
and applicable to subjects of all classes we will consider in some de- 
tail the proper methods of handling each class of copy in order to 
obtain the best results. 

Subjects in black and white embrace an extensive and varied field 
which includes charts, graphs, maps, pen and ink and pencil sketches, 
wood and steel engravings, etchings and half-tone reproductions. 
The photography of such subjects, while quite simple in itself, de- 
mands precise painstaking attention to every detail if the best results 
are to be obtained. 

While the subject of lenses has already received attention, it may 
be well to remark at this point that in dealing with line work the best 
corrected objective is none too good; especially is this the case when 
dealing with subjects containing very fine detail or when the degree 
of reduction is considerable. In such cases the absence of zonal ab- 
berations and coma is particularly desirable and the process anastig- 
mat is well worth its additional cost where work of this type must be 
done. 

The plates required for handling this class of copy are known as 
Contrast, Process or Photo-Mechanical plates and are made to give 
a very high degree of contrast. Wet collodion is still unsurpassed for 
line work but its use is beyond the capabilities of most workers, but 
with care all that can be done with collodion can also be done with 
eelatine although it must be admitted that a satisfactory result from 


062 PHOTOGRAPHY 


difficult originals is more difficult to secure with gelatine than with 
collodion. Rapid plates as used for general work cannot be used for 
this purpose as they do not have a sufficiently fine grain and are un- 
able to give the great density combined with absolutely clear lines 
which is required for this class of copy. Plates of the process type 
suitable for line work are all comparatively slow and range in speed 
from about Watkins 15 to Watkins 45 but work very free from fog 
and readily give great contrast and density. Typical plates of this 
class are Cramer Contrast, Seed Process, Eastman Process film, Im- 
perial Process, Wellington Ortho Process, Ilford Process and Half- 
Tone, Barnet Process and Gevaert Process. 

Development of Process Plates.—lhe development of process 
plates is best conducted by inspection using a concentrated hydro- 
chinon or glycin developer. The following is considered the best 
formula for obtaining the maximum contrast: 


A. Sodium’ bisulphite.. $5 62 cen 2. 5 on ee 375 gr. 25 gm. 
Hydrochinon 0. hei ee ods os 0 eas 25 gm. 
Potassium: bromides)... 2.303 (2 ee 375—s grr. 25 gm. 
Water: to: make. aoe. Shae ‘92> :RiCs 1000 cc. 

B. Caustic® sodas iici.02 4a oe oi eee ee 1% oz. 45 gm. 
Water to make. ...5..00:.5 25 chico ee 32° og, 1000 cc. 


For use take equal parts of A and B. The developer will not keep 
when mixed and a separate batch should be used for each plate. An- 
other formula which has good keeping qualities and gives good con- 
trast is as follows: , 


Hydrochinon 2.43.2) <2. Sea's fas ae 130 gr. 15 gm. 
Sodium ‘sulphite: (dry) 0.0.4.5), Ue oe 3 OZ. 150 gm, 
Formialite -* s,s 44 sq: adbeast cminpn wl’ 0 gee pale ae ae 20 cc. 
Water -to make... 7.0.5.5 en te 20 OZ. 1000 cc. 


This is one of the few cases in photography in which the author 
prefers development by inspection to the time and temperature or fac- 
torial methods. For one thing, a comparatively bright light may be 
used with safety so that there is no difficulty in judging the appear- 
ance of the negative. As process plates fix back considerably, de- 
velopment must be carried just as far as possible without causing the 
delicate lines to veil over. A slight veiling, noticeable towards the 
close of development, may be disregarded, as it will disappear in the 
fixing bath. If development is carried as far as possible without pro- 
ducing fog, the density will be all that is desired, unless exposure has 


a 


COPYING 563 


been insufficient. If the exposure has been insufficient the negative 
will lack density when removed from the fixing bath, even though 
the density appeared to be sufficient when development was concluded. 
On the other hand should the lines begin to veil over in the early 
stages of development before the requisite density is obtained, over ex- 
posure is indicated. In fact, exposure to suit the original is the key 
to the whole problem, provided the proper plate and developer are 
used and development is carried to the limit. 

Iexcept with weak originals, or through faulty exposure or develop- 
ment, intensification will not be required. When intensification is 
necessary Monkhoven’s silver-cyanide method, lead or copper are 
suitable for the purpose. 

For the print glossy papers are generally used, especially if there 
is an abundance of small detail. It may be noted that a hard, vig- 
orous paper gives a cleaner-cut black line than the normal or soft 
varieties. 

Copying Photographs or Like Subjects in Monochrome.—Here the 
object is to reproduce the various tones of the original as correctly 
as possible. A slight loss is inevitable, particularly at the ends of the 
scale of gradation, but if the copy is well made the loss should be small 
and practically indistinguishable. Especial care must be taken to 
minimize the grain of the original, particularly if the surface is matt 
or rough. The apparatus described earlier in this chapter will be of 
great assistance in this respect. Under exposure and forced develop- 
ment, or the use of a contrast working plate, accentuate any tendency 
to “graininess”? and in such cases it is well to expose fully and 
shorten the time of development somewhat. When very contrasty 
originals must be copied the use of an ultra rapid plate will assist ma- 
terially in toning down the extremes of contrast, but for general 


work high speed plates are not to be advised and better results will be 


secured by the use of comparatively slow plates ranging from Watkins 
50 to Watkins 150. Plates of this character, made especially for this 
class of copying, are made by practically all manufacturers. <A great 
deal depends upon the exposure and only experience can show what 
is required in this respect. A careful record of all experiments and 
the results secured will assist materialiy in estimating exposures as 
will standardization of all controllable factors along the lines which 
have already been indicated. 

Development must be conducted with judgment, and cannot well 


564 PHOTOGRAPHY 


be made uniform for all subjects. The strong contrasts of some 
prints, particularly those of a non-actinic color, require to be softened, 
while flat bluish-black originals should be developed further in order 
to secure sufficient contrast. In many cases, such alterations can be 
made by judicious choice of the grade of paper in printing, but at 
times intensification or reduction may be necessary to secure the 
proper contrast. 

The Photography of Colored Objects.—In dealing with subjects 
of this class orthochromatic methods are necessary. The subject of 
plates and filters and their action has been treated in a former chapter 
so that at this point we will mention only some practical points con- 
nected with the photography of colored objects and for further in- 
formation the student is referred to Mees, Photography of Colored 
Objects. 

_ In general, it may be said that the panchromatic plate is preferable 
to orthochromatic for all subjects involving color. If desensitizers, 
or time and temperature methods of development, are used they may 
be handled with no more difficulty than attends the manipulation of 
other plates, while their enhanced color sensitiveness to all colors, — 
which allows of shorter exposures for the same degree of color cor- 
rection, their red sensitiveness and the fact that they enable one to 
standardize the matter of plates are all important points in their 
favor. 

While in ordinary work, where the subject is at a considerable dis- 
tance from the lens, the shift in the plane of sharp focus due to the 
use of a filter is comparatively small and may in many cases be ig- 
nored, when copying the matter becomes of considerable importance 
and filters of the finest optical properties become imperative. Filters 
supplied for ordinary photographic work are cemented in specially 
selected glass and are sufficiently near to a plane surface and parallel 
on both sides not to affect the definition of a lens when used for dis- 
tant objects. But, whereas the rays from distant objects are prac- 
tically parallel, those from near objects, as in copying, are quite 
divergent and if the filter is not both plane and parallel, or in other 
words is wedge-shaped, it will have the effect of a prism and destroy 
the finer corrections of the objective with which it is used. The 
longer the focal length of the objective the more accurate the filter 
must be in these respects. A filter which would pass muster with a 
six-inch lens might be practically useless for one of twelve inches 


COPYING 565 


focal length. For critical copying from colored objects the filters 
should be cemented in optical flats which are polished and tested with 
the same accuracy as the highest grade lenses. Gelatine filters are 
quite satisfactory but it is hard to keep them clean. The filter may 
be placed either in front of or behind the lens; the latter is the best 
position as there is less danger of flare, or flare spot, when the filter 
is behind rather than before the lens. : 

For photographing paintings, water color work and crayon a pan- 
chromatic plate is necessary. The K, or fully correcting filter advised 
by the manufacturer of the particular brand of plates employed is re- 
quired for nearly all subjects as usually it is desired to give an exact 
color rendering of the original. There are cases, however, where it 
is necessary to overcorrect some color at the expense of another and 
for this purpose a contrast filter is necessary. This should always be 
done with caution, however, and whenever possible orthochromatic 
methods should not be departed from. 

Reflections from the surface of paintings are very hard to avoid as 
they are different from those from a flat surface. The light is re- 
flected from the irregularities in the surface where the paint has been 
laid on thick and not from one definite plane as is the case with photo- 
graphs, etc. Placing the painting at a considerable distance from the 
source of light and cutting out all bright objects in front are about 
the only methods of avoiding these reflections. ‘The angle at which 
the light strikes the surface is important but the best angle can only be 
found by trial and error for each particular subject. ‘Tilting the pic- 
ture forward is often of advantage; the swing back being used to 
correct the attendant distortion. Daylight is the best light for copy- 
ing paintings; artificial light never seems to give quite the proper ef- 
fect artistically. The exposure varies with the character of the pic- 
ture; a dark old Master requires very much more time than one of 
the late productions of the Japanese school. There is a tendency 
for photographs of paintings to have too much contrast and to coun- 
teract this tendency a full exposure should be given and the usual time 
of development shortened somewhat. Only experience can teach the 
worker how to handle subjects of this very difficult class. 

Silver prints toned sepia, gum and oil prints and transfers all re- 
quire much the same treatment as paintings unless they are in black or 
blue-black. If a sepia-toned silver print is copied on an ordinary plate 
.we invariably secure too much contrast unless the original happens to 


566 PHOTOGRAPHY 


be flat and lacking in contrast. To secure the best results from sepia- 
toned silver prints a panchromatic plate with a K, or similar fully 
correcting filter should be used. Gum-bichromate, carbon or oil prints 
and transfers in green, brown, red and similar colors should receive 
like treatment. 

Blue-prints, violet or blue typewriting can be successfully Aiostts by 
the use of a panchromatic plate together with a deep red filter. Ina 
similar manner stained prints may be copied and the stain removed by 
the selection of filters appropriate to the color of the stain. This 
matter has already been touched upon in a former chapter. 

Photography of Small Objects in the Studio—lIt is perhaps well 
that we devote a few lines to this subject as the photography of such 
articles as knives, watches, small packaged articles, etc., forms a rather 
large part of the business of a commercial photographer. 

In all such work it is a great advantage to be able to use the camera 
vertically as the articles then remain in whatever position they are 
placed and it is unnecessary to attach them firmly to a support. 
Furthermore it is much simpler to make use of the methods to be de- 
scribed for obtaining suitable backgrounds without laborious after- 


work such as blocking out with opaque or etching away what is un- 


desirable. 

White grounds are easily obtained and all necessity for blocking 
obviated by the use of a “light box” as shown in Fig. 234. This is 
simply an ordinary box with the top and front removed, lined inside 
with a white blotter or coated with aluminum paint and covered with a 
piece of ground-glass (ground side up) for holding the article to be 
copied. The reflecting surface may be sloped so as to catch the light 
to the best advantage or, if the volume of such work justifies it, day- 
light may be replaced by electric bulbs placed so as to spit! illumi- 
nate the ground-glass above. 

Light objects appear to better advantage against a black background. 


eS eS ee ee ee 


Black velvet is suitable for this purpose but black paper cannot be 


used as it does not give a pure uniform black owing to its texture. 
The best results, however, are secured by a “dark box” which is 
exactly opposite to the “light box” previously described. This is 
merely a large and deep box painted black on the inside or lined with 
black paper, the top of which is covered except for an aperture just 
large enough for the size of background desired. The subject is 
placed upon a piece of clear glass over this black hole and the exposure 
made. 


COPYING 567 


Sometimes the use of clear glass gives rise to reflections and in such 
cases it will be necessary to fasten to the under side of the camera a 
large sheet of black cardboard with a hole cut to accommodate the 
lens. When this is ineffective we must resort to a hood of tissue paper 
or tracing cloth to eliminate all direct light. A cone of tissue paper or 


(Photos courtesy of D. J. Pratt) 
Fic. 234. Method of Securing White or Black Backgrounds 


tracing cloth is made to enclose the space between the lens and the 
background on which the object is placed so that all direct light is pre- 
vented from reaching the glass and reflections removed. The use of 
this hood of tracing cloth lengthens exposure to a certain extent but 
reflections which cannot otherwise be removed will yield to this treat- 
ment which in such cases is well worth the trouble which it involves. 


CHAPTER XXVI 


NATURAL COLOR PHOTOGRAPHY 


Introduction.—Joseph Nicephore Neipce writing in May, 1816, to 
his brother Claude then residing near Kew in England states that one 


of the problems which he has yet to solve and one which will receive © 


his attention in the future is the fixation of the colors by which he 
probably meant the reproduction of objects in their natural colors. 
This problem, however, that patient investigator was not destined to 
solve nor have we who live more than a century later been entirely 
successful. While the subject has attracted the attention of some of 
the foremost scientists and much has been accomplished we are still 
far from a practical process of color photography on paper. We 
have, nevertheless, made substantial progress and great as the obstacles 
may now appear there is every reason to beneye. that the problem will 
be solved eventually. 

It is the purpose of this chapter to record the work which has been 
done on the subject and to describe the processes now available for 
photography in natural colors. 

Processes of Direct Color Photography.—Seebeck as far back as 


1810 found that silver chloride when exposed to the rays of the spec-. 


trum partook slightly of the colors themselves and Edmond Becquerel 
in 1844 reproduced the seven principal colors of the spectrum on a 
Daguerreotype plate which had been so treated as to form a photo- 
chloride of silver (Ag : Cl) which has the property of giving a partial 
reproduction of color which, however, cannot be fixed. Similar proc- 
esses were described by Robert Hunt, Sir John Herschel, J. W. 
Draper, Neipce de St. Victor, G. Wharton Simpson and Poitevin. who 
was able to secure color prints from colored glass transparencies on 
paper prepared with silver photo-chloride. ‘These prints, however, 
like all prints involving the use of a photo-chloride of silver could not 
be fixed while the paper was much too insensitive to allow: it to a used 
in the camera. 

Natural color photography along mito lines sao somewhat 
better with mixtures of light-sensitive dyes; that is, dyes which fade 
out to colorless substances. A dye is decomposed only by the light 

568 


| 
a 
| 
d 
q 
| 


. 
eye 


NATURAL COLOR PHOTOGRAPHY 569 


which it absorbs (Grothius-Draper Law) which color is complementary 
to its own color. Certain aniline dyes bleach comparatively rapidly in 
light, hence if three such dyes are chosen so as to form the three 
fundamental colors red, green and blue-violet and these are coated on 
paper in three separate layers and the whole exposed to a colored 
object, in red light the green and blue dyes will bleach out, leaving the 
red; in the same way in blue light, blue will be left as red and green 
will bleach out and in the case of green, red and blue will bleach out 
while with colors which are mixtures of these each will be bleached in 
direct proportion to the amount of the fundamental colors present. 

Processes based on this principle were suggested by Cros in 1881, 
Liesegang in 1889, Ives in 1891, Vallot in 1895, Neuhaus in 1902, 
Worel in 1902, Szczepanik and Dr. J. H. Smith in 1907, 1908 and 
IQIO. 

Despite the apparent simplicity of the process it has never furnished 
a satisfactory solution to the problem of natural color photography. 
To secure three dyes having the proper colors and of identical light 
sensitiveness is not easy and this difficulty together with that of pre- 
venting further bleaching of the dyes after exposure and the com- 
paratively low sensitiveness of such mixtures has prevented such 
methods from progressing beyond the experimental stage. 

Direct Color Photography by Processes of Light-Interference.— 
To understand the ingenious process of color photography worked out 
by Professor Lippmann of Paris in 1897 it is necessary to review 
briefly the nature of light and the principle of light-interference. The 
generally accepted theory of light is that it is a wave motion in an 
elastic medium known as the ether and is propagated in waves of the 
transverse type. Suppose two wave motions are made to go in op- 
posite directions by reflection from a highly reflecting surface. Inter- 
ference will then occur between the incident and reflected waves, re- 
sulting in the formation of standing waves. ‘Thus at intervals equal 
to half the wave-length there will be alternate maxima and minima of 
light intensity. Now if we place in contact with this highly reflect- 
ing surface a “ grainless”’ and transparent emulsion of silver halide, 
on exposure to light of a definite wave-length the chemical action will 
be distributed in a number of layers, the maximum action taking place 
at the crests of the waves and the minimum action at the nodes of the 
standing waves. On development the layers of exposed silver halide 
are reduced to the metallic state. Thus there will be formed for each 
‘color a set of mirrors the separation of which is exactly equal to one 
half the wave-length of the light by which they were produced. 


570 PHOTOGRAPHY 


When the image is examined perpendicularly by reflected light, the 
light which is reflected to the eye is the sum of the reflections from 
these elementary mirrors. The distance between these, however, is 
one half of the wave-length of the light by which they were produced, 
therefore when viewed in white light the colors which are not of the 
proper wave-length are destroyed by interference so that the light 
reflected from that portion of the image corresponds in color to that 
which produced the image. 

Lippmann’s method was to expose a specially prepared fine-grained, 
transparent emulsion of silver chloride in contact with a bath of 
mercury which reflected back into the emulsion the waves of light 
which reached it, thus setting up in the sensitive film the phenomenon 
of interference described above. While the process is extremely in- 
teresting as the verification of certain theories of light and color it is 
little more than a laboratory experiment. The fine-grained, trans- 
parent emulsion employed must be prepared by the worker and special 
equipment is necessary in order that it may be exposed in contact with 
a surface of mercury. Furthermore only the pure colors of the spec- 
trum are accurately reproduced; with the ordinary mixed colors of 
nature the rendering is not so good. Lastly, a lengthy exposure is 
necessary and the results cannot be duplicated. The process therefore 
is little more than an interesting laboratory experiment. 

Natural Color Photography by Trichromatic Methods.—Promis- 
ing as such processes of direct color photography may appear theoreti- 
cally it is with indirect methods involving the separate registration of 
the three fundamental color-sensations and their subsequent recom- 
bination that the greatest progress has been made. All such methods 
are based upon the discovery by Thomas Young in 1807 of the fact 
that all color perception is the result of three fundamental color- 
sensations singly or in various combinations and proportions. ‘That is 
to say, all of the colors observed in nature are formed by the mixture 
in various proportions of the three fundamental;. or primary, colors, 
red, green and blue. These three fundamental colors cannot be pro- 
duced by the admixture of any other colors but from them any color 
in nature may be matched including white, which is a mixture of all 
three in equal parts. Hence if on one plate we record the red sensa- 
tion of the subject by making the exposure through a filter trans- 
mitting red only, on another plate the green sensation by the use of a 
green filter and on a third plate the blue sensation by the use of a 
blue filter, we have recorded the three fundamental sensations, which 


" 
ee ee 


NATURAL COLOR PHOTOGRAPHY 571 


singly and in various combinations comprise all the colors of the sub- 
ject. Recombination of the three-color sensation records may be ac- 
complished in several ways: projection of transparencies in a triple 
lantern, in various viewing instruments to be described later and by 
superimposed layers of pigments of the proper colors. 

Such, in brief, is the basis of the three-color processes of natural 
color photography, the principles of which were first clearly realized 
by Professor James Clerk Maxwell in 1861. 

Making the Three Color-Sensation Negatives——The three nega- 
tives may be made with an ordinary camera by making three separate 
exposures and changing the plate-holders and filters between each ex- 
posure. With many subjects, however, there is the liability of move- 
ment between exposures while there is always the danger of shifting 
the camera slightly or upsetting the focus, thus destroying the very 
necessary correspondence of all three negatives. Much better is the 
use of a repeating back by means of which it is possible to make all 
three exposures in fairly quick succession. ‘To avoid trouble from the 
movement of the subject between exposures, cameras have been devised 
_which make all three negatives at the same time. Owing to the fact 
that all three negatives must be identical in size and detail it is im- 
possible to use three separate lenses side by side for the difference in 
the viewpoint of the separated lenses would destroy the exact corre- 
spondence of the three negatives. Hence, only one lens may be used 
(unless we are willing to content ourselves with very distant objects in 
which case the effect of a small difference in viewpoint is not so 
noticeable), and the three images formed by means of transparent re- 
flectors or prisms. | 

It would take us too far afield to consider at any length the various 
cameras which have been designed for making three-color negatives 
at a single exposure. There are plenty of them as may be seen by 
reference to Professor E. J. Wall’s History of Three-Color Photog- 
raphy which is the most authoritative and complete work on the sub- 
ject of trichromatic photography. 

One of the most successful one-exposure, three-color cameras em- 
ploying prism separation is that constructed by Sanger Shepherd and 
Company of London after Ives’ British Patent 12,181 of 1900. The 
two outer sections of the image (Fig. 235) are diverted by the two 
rhomboidal prisms and form the red and blue negatives while the clear 
space between the prisms forms a direct image which records the green 
sensation. ‘The stero error is very small with this construction and is 
unimportant for all ordinary subjects. 


572 PHOTOGRAPHY 


Cameras using reflectors form the biggest class and while such a 
construction was first described by Ch. Cros in 1871, to Mr. F. E. Ives 
belongs the credit for having determined the factors necessary to make 
ita success. Besides Mr. Ives a large number of other workers have 


Fic. 235. Sanger Shepherd Three-Color Camera 


described various forms of one-exposure, three-color cameras using ~ 


either one or two mirrors references to which may be found in the 
bibliography or in Professor Wall’s History of Three-Color Photog- 
raphy. We illustrate in Fig. 236 a camera of this type designed by 


Red | Plate 
[Red | Filter] 


LQ" ley 
> i 
=p 


940/d > 


Fic. 236. Butler’s One-Exposure, Three-Color Camera 


Mr. E. T. Butler. Part of the light entering the lens on the right is 
reflected by the first mirror and after passing through the red filter 
forms the red-sensation negative. The light passing through the first 
reflector strikes the second reflector which reflects a portion of it to 


—— se oa, pe 


/ 
i 
: 

‘. 
4 

r 
7 
] 


a ae 


NATURAL COLOR PHOTOGRAPHY 573 


form the blue negative while the light which passes through this re- 
flector forms the green-sensation negative. It is necessary that all 
three negatives be of the same size and sharpness, hence the distance 
traversed by the light rays must be the same for all three images. 
Furthermore it is essential that the reflectors do not produce a double 
image to overcome which it is necessary to cover the back of the glass 
with colored gelatine. This color must be the minus color of the 
taking screen and since the first reflected image forms the red sensa- 
tion the gelatine coating on the back of this reflector must be minus 
red or blue-green while that of the second reflector must be minus blue 
or yellow. 

In order to avoid the necessity for a one-exposure, three-color 
camera, Louis Ducos du Hauron suggested a tri-pack, the three plates 


or films being bound up together with their respective filters in be- 


tween. Critical sharpness, however, is impossible with such an ar- 
rangement as it is impossible to bring the three emulsions sufficiently 
close together. This alone might not be a grave objection in certain 
cases and might be disregarded but for another more serious difficulty. 
The light which passes through the first plate is diffused by the particles 
of silver salt making it impossible to secure a halation-free image 
on the second plate. Halation is of course even more pronounced on 
the third plate since in this case the light has been scattered by two 
emulsions. In addition there is the difficulty of adjusting the speeds 
of the three plates so that each will be properly exposed in the same 
time. In practice, therefore, attempts to develop such methods have 
not been very successful. 

Additive and Subtractive Three-Color Photography.—The color- 
sensation negative records by density the presence of that particular 
color in the subject; i.e. the red-sensation negative records the red of 
the subject in terms of greater or lesser density according to the 
amount of red present in the various portions of the subject. A 
positive transparency from this negative will reproduce the red sen- 
sation by means of its clearer parts. The parts of the subject con- 
taining the purest red will be represented by clear glass, those parts 
with some red by a medium density while those parts containing no 
red whatever will be of maximum density. Now if this transparency 
is viewed in red light it will reproduce the red sensation of the original 
subject. In like manner the blue and green transparencies will, when 
viewed in blue and green light, reproduce the respective color sensa- 


tions of the original subject. 


38 


574 } PHOTOGRAPHY 


The three records may now be combined and the natural colors of 
the subject reconstructed by placing each transparency with its proper 
filter in a viewing instrument constructed like the one-exposure, three- 
color cameras already considered. This procedure, like most others 
in three-color photography, was first developed by Louis Ducos du 
Hauron. It reached its highest development in the hands of Mr. F. E. 
Ives whose Kromskop has never been surpassed for absolute fidelity 
in color reproduction. 

It is to be noted that in this case colored light is added to colored 
light. We start with colored light from which we produce white by 
addition. Hence such processes are termed additive processes. 


Red. 


Green SWhite 


Bites 


To recombine the three-color sensations on paper or in a single 
transparency it is necessary to superimpose three separate images of 
the proper colors. The white paper on which we place our colored 
images reflects all three primary colors, red, green and blue, which 
as we know, form white. Now when we print from the red-sensa- 
tion negative we are printing from the thinner parts, or those parts 
which represent the absence of red in the subject. Hence the red- 
sensation negative must be printed, not in red, but in a color which 
completely absorbs all red. But while red is absent either one or both 
of the two other primary colors may have been present in this portion 
of the subject. The color of the image, then, must be such that it not 


only absorbs red but reflects green and blue. It will, therefore, be a. 


minus red or blue-green. The red-sensation negative is thus printed 
in minus red or blue-green; the green negative in minus green or 
magenta which absorbs green and reflects blue and red while the blue- 
sensation negative is printed in minus blue or yellow which reflects red 
and green but absorbs blue. ) 

Superimposed in full strength these colors absorb all color and the 
result is either black or gray according to the amount of light reflected. 
Intermediate colors are produced by the mixture in various propor- 
tions of the three fundamental colors while the total absence of color 
will produce white, since this is the color of the paper base. 

It will be observed that in this case we start with white light from 


, P 3 nel 
oe ae ee ee 4 


NATURAL COLOR PHOTOGRAPHY 575 


/ 
which we produce color by subtracting various colors, hence such 
processes are known as subtractive methods. 


/Red—Minus red or blue-green 
eee vinus green or magenta 
Blue——Minus blue or yellow 


Subtractive Printing Processes.—The principle of the subtractive 
method has been developed in a wide variety of processes. But few 
of these, however, are generally employed and these now only in cer- 
tain quarters, the development of the screen-plate processes having 
largely killed the interest which was shown in such methods a few 
years ago. ‘The three-color images for the subtractive processes have 
been produced by means of trichromatic carbon tissues and lately by 
three-color carbro; by the production of dye images by mordanting 
methods, by the transference of dyes or the relief or imbibition 
process represented by the Pinatype method; by three-color gum- 
bichromate and by the toning of silver images. While prints by these | 
methods are often quite pleasing from the artistic standpoint there 
is a tendency, more noticeable in some processes than cthers, to dull 
and imperfect colors lacking in brilliancy and transparency owing to 
the depth of the three superimposed pigment or dye images. This 
together with the very practical difficulties involved in producing the 
three images and in properly superimposing them, the complexity of 
the process and the care and delicacy demanded at every stage, places 
the process beyond the possibilities of the average worker, hence such 
methods have failed to make much headway. 

Multi-Color Screen Plates.—In just the same way that a painter 
may secure a certain color by the juxtaposition of ‘dabs of pigment 
of two colors which when viewed at a distance merge to form a single 
color, so it is possible to secure on a single plate all three color-sen- 
sation records by eniploying in place of the usual solid color filter a 
multi-color screen composed of a large number of small color screens 
evenly distributed and so small as to be practically invisible. 

The multi-color screen was the conception of Louis Ducos du 
Hauron whose patent of 1868 suggested that a sensitive plate be ex- 
posed behind a screen composed of fine parallel lines, red, green and 
blue. The red lines collectively record the red sensation of the sub- 
ject, while in like manner the green and blue lines collectively record 
the green and blue sensations respectively, so that all three funda- 
mental color records are secured on a single plate. Consequently 


576 PHOTOGRAPHY 


when a positive from the original negative is placed in contact with 
the multi-color screen in the position occupied by the negative, so that 
the lines in the positive recording the red sensation are behind the red 
lines of the multi-color screen, the colors of the subject become visible, 
the same principle being brought into play as in the viewing camera. 
The multi-color screen plate process of color photography is thus an 
additive method. 

While Louis Ducos du Hauron was the first to develop the idea 
of a multi-color screen, the practical development of the method is 
largely due the work of Professor Joly of Dublin and James Mc- 
Donough of Chicago. The former was granted a patent (B. P. 7743 
of 1893, 13,196 of 1894) for a screen plate with parallel red, green 
and blue lines having a width of about 0.12 mm. (1/200 inch).. His 
patents together with those of an American inventor James Mc- 
Donough of Chicago, who had devised a similar screen plate but with 
finer lines, were acquired by a syndicate which placed the process on 
the market but owing to the difficulties met with in manufacturing the 
screen plates economically it soon ceased to exist. In succeeding years 
a large number of patents have been taken out for multi-color screens 
employing not only ruled lines but various geometrical shapes such 
as squares, rectangles, circles, etc. As these, with one exception, are 
no longer on the market we will not linger to consider them but pass 
directly to the second type of screen plate in which the color screens 
are distributed at random and do not form a definite geometrical pat- 
tern as in the two examples just quoted. The two most conspicuous 
examples of this type of screen plate are the Lumiére Autochrome in- 
troduced by A. and L. Lumiére of Lyons in 1907 and the Agfa Color 
Plate. 

With a multi-color screen of a definite geometrical pattern the 
screen may be separate from the sensitive plate; the positive trans- 
parency from the negative made behind such a screen being placed 
in register with another similar screen for viewing purposes. With 
the second type of multi-color screen, known as the mosaic screen 
plate, this is impossible and the negative image obtained by develop- 
ment must be chemically reversed. The separate screen-plate method 
permits of unlimited duplication as one need only make as many posi- 
tive transparencies as required. With the mosaic screen plate, how- 
ever, duplicates can only be made by rephotographing the original and 
at the expense of some loss of brilliancy of coloring. The duplicating 


NATURAL COLOR PHOTOGRAPHY 577 


method is perhaps the simplest for the beginner in color photography 
but both processes are well within the possibilities of the amateur who 
is already conversant with the principles of ordinary photography. As 
regards the faithfulness of color reproduction there is little difference 
between the two methods; the colors as reproduced by the mosaic 
screen plate, however, are supposed to be somewhat softer and with 
less tendency towards glaring color than the duplicating method. 
But this difference is so slight as to be of little if any importance. 

The Autochrome Plate—TJhe Autochrome multi-color screen is 
an example of the mosaic screen and was the first of such to meet 
with success. The method of preparation is most ingenious. The 
colored screens are composed of a particular form of starch grains 
ranging in size from 10/1000 to 15/1000 of a millimeter (.0024 
inch). Separate lots of these grains are dyed orange, red, green 
and blue-violet. These are then mixed in such proportions that the 
result shows no predominating color and this mixture is spread over 
the glass plates. The gaps between the grains are filled in by means 
of extremely fine charcoal dust, after which a layer of waterproof 
varnish is applied so as to separate the screen from the emulsion 
which is coated on top of it. 

As these starch grains number 6000 to the square millimeter 
(about 4% million to the square inch) they are invisible to the 
eye. When observed with the microscope at a magnification of about 
125 times, the appearance of the screen is illustrated in Fig. 237 in 
which the darker circles represent the blue-colored grains, the half- 
tone circles the red grains and the lightest circles the green grains. 
From this it is evident that the grains of any given color are very 
evenly distributed throughout the screen. This is of course neces- 
sary for the opposite state of affairs would result in color patches 
which would render proper color reproduction impossible. 

Over this multi-color screen is coated a thin, highly color-sensitive 
emulsion. As it is impossible to make this emulsion equally sensitive 
to all three colors, it is necessary to compensate for this deficiency by 
means of a filter applied to the lens. As the absorption of light by 
the multi-color screen is considerable (the Autochrome screen ab- 
~sorbs about 92.5 per cent of the incident light, which amount is still 
further increased by the compensating filter which must be employed), 
the working speed of the plate is much less than the ordinary plate or 
film and is about 4 Watkins or 2.4 H. and D. Very rapid exposures 


578 PHOTOGRAPHY 


are, therefore, impossible-unless the plates are hypersensitized or flash- 
light is employed. The former operation is not one which should be 
attempted by the novice. 

The Compensating Filter.—The filter supplied by the Lumieres 
is calculated for use with average daylight. As the spectral com- 
position of daylight is never constant, however, and moreover varies 
greatly in different localities, it is obvious that any single filter is at 


Fig. 237. Autochrome Screen. 125 


best a compromise. With the vast majority of subjects, however, and 
in the temperate zones the filter supplied by the manufacturers is 
entirely satisfactory but certain subjects which are unusually strong 


in blue and violet rays require a deeper filter. Thus in early morning 


or late afternoon when the light is rich in color, subjects including far 
distances show marked blueness in these portions. Likewise in ma- 
rine photography or with subjects having wet surfaces, snow scenes, 
etc., excessive blueness of tone is often observed. Achille Carrara 
has found that the intense blue of the Italian skies and lakes leads to 
excessive blueness in the finished result. 

In such cases it is necessary to employ a filter absorbing a greater 
amount of ultra violet than the standard filter. For this purpose an 
additional screen of esculine or Filter Yellow K may be employed. 


Or the usual filter may be supplemented with a Wratten K1 filter for 


a part of the exposure, or, in extreme cases, for the entire exposure. 


? 


NATURAL COLOR PHOTOGRAPHY 579 


The use of filters which absorb too much ultra violet leads to a 
prevailing yellow tint in the completed transparency. 

Special filters are required for artificial light sources. These may 
be obtained on special order from the manufacturers. 

Handling and Exposure of the Autochrome Plate—The Auto- 
chrome emulsion being sensitive to all colors must be handled either 
in total darkness or by a safelight formed of the Virida papers of the 
makers. As it is not a difficult matter to load plate-holders in total 
darkness when one has become familiar with the operation, it is ad- 
visable to place the plates into the holders in total darkness. A gen- 
eral greenish tint in the finished positive may often be traced to the 
ise of an unsafe light or to excessive exposure of the plate to the 
Virida light when loading. 

Since the multi-color screen must be in front of the sensitive emul- 
sion during exposure, the glass side of the plate is placed towards the 
lens. The sensitive film being very delicate it is protected by a piece 
of cardboard which should not be separated from the plate until the 
moment of development. Otherwise the delicate film may be damaged 
and the plate soon develops fog. 

Before inserting the slide it is well to brush off any adhering par- 
ticles of dust or other substances which may be adhering to the glass 
side of the plate in order that such may not produce a plentiful crop 
of black spots in the finished result. 

As the plate is exposed through the glass a correction is necessary 
when focusing. If the filter is placed behind the lens this correction 
is made automatically and this is the proper method to employ with a 
fixed focus camera or those focusing by scale. If the filter is placed 
before the lens the ground-glass may be reversed so that the ground 
side is on the outside. One may move the lens back a distance equal 
to the thickness of the Autochrome plate (1.8 mm. or %4 inch) or em- 
ploy a Zeiss Ducar filter which automatically compensates for the 
thickness of the plate and allows the same camera to be used for either 
ordinary or color work without any inconvenience whatsoever. 

Exposure.—As in ordinary photography, and to an even greater de- 
eree, success in color photography with screen plates is dependent upon 
correct exposure. While ordinary plates and films have considerable 
latitude in exposure, so that one or two times more will still produce 
a usable negative, the margin of error is very small in color photog- 
raphy, only a few per cent at the most, and correct color rendering 


580 PHOTOGRAPHY 


cannot be obtained without the proper exposure. Numerous tables 
have been published for the calculation of exposures for the Auto- 
chrome plate but these do little more than indicate the approximate ex- 
_ posure and the factors on which their successful use depends are very 
difficult to estimate accurately, so the use of tables is not very satis- 
factory. The only satisfactory method lies in the use of an actinom- 
eter such as the Watkins or Wynne to which reference has already 
been made in a previous chapter. Special color plate meters are 
supplied by the makers and the use of these is preferable to the regu- 
lar form because those designed particularly for color work are pro- 
vided with scales which take into consideration the failure of the rec- 
iprocity law which occurs with plates of very low sensitiveness as 
the Autochrome plate. Owing to the low working speed of the Auto- 
chrome plate, the reciprocity law according to which exposure is the 
product of time and intensity, which are inversely proportional, does 
not hold. Therefore, in working in feeble light or with a small dia- 
phragm the increase in exposure is more than that which would be 
indicated by the law. According to M. Fauchet, reduction of inten- 
sity by one half increases the exposure by about 2.25. 
Development.—In 1907 when the Autochrome plate was introduced 
a pyro-ammonia developer was recommended. This, however, has 


subsequently been replaced by one of metoquinone and while many of 


the older workers prefer the former, metoquinone is the best for the 
novice. 


The formula is as follows: 


Metoquinone « «0055.0 4 ..0k ba au vas Clee YY oz. I5 gm. 
Sodium sulphite (dry }-.. ..05 so 3% oz. 100 gm. 
Ammonia 920 (22° Baume) 0.2. 225.9 eee 657° nO 
Potassium bromntide: .$ 2.0 0¢a000 vent go gr. 6 gm. 
Distilled water to... isn d.50er es oe eee $0 208) 1000 cc. 


For time development dilute one part of the above concentrated 


stock solution with four parts of water and develop exactly 2% min- ~ 


utes at 60° F. The Watkins Meter Company supply a special ther- 
mometer which shows by the height of the mercury the time for de- 
velopment at any temperature. 

Development for a fixed time is suitable only for plates which have 
been correctly exposed. For all others, preference should be given to 


a controlled method based upon the time of appearance of the image. ; 
To develop by this method one begins development in a diluted de- — 


ae ee eS se ae 


NATURAL COLOR PHOTOGRAPHY 581 


veloper, taking the time of appearance of the image in this solution. 
On the appearance of the outlines of the image (the sky being dis- 
regarded) the developing solution is strengthened by the addition of a 
certain amount of concentrated metoquinone developer according to 
the time required for the first appearance of the image. For a plate 
up to 4x6 inches in size one may begin development in a solution 
composed as follows: 


SRT ie oa vce aks Me eee ve 80 cc. 24 oz. 
Concentrated metoquinone developer.............. 5 cc. 85 min. 


The following table then shows the amount of concentrated developer 
to be added upon the appearance of the image and the total duration 
of development. 


Appearance of Outlines of Im- Quantity of Developer A to Total Duration of Develop- 
age (Disregarding Sky) Add en Appearance of ment From Immer- 
Atter Immersion. First Outlines. sion of Plate. 
Seconds Minutes. Seconds. 
I2to 14 15 c.c.s. (4 02.) I 15 
15 to 17 do. do. I 45 
1to2r. do. do. 2 15 
a2 10 27 do. do. 3 fe) 
23 t0:33 do. do. = 30 
S410 30°, do. do. ae. 30 
_ Extreme 40 to 47 A5 c.c. (1% ozs.) z O 
under-exposure { Above 47 45 c.c. (13 ozs.) 4 O 


(If it is thought desirable for any reason to use a larger volume of 
developing solution all the quantities given should be increased ac- 
cordingly. ) 

M. F. Dillaye recommends that the exact time of appearance be taken 
by transmitted light and then watching for the moment at which the 
image, which first appears as a negative, seems completely extinguished, 
the whole plate presenting the appearance of an even, diffused. density. 
It is at the moment at which this occurs that development should be 
stopped. If development is continued the image appears as a positive 
and will be over developed. This method while possibly practical for 
the advanced worker is not one which the novice should attempt. 

One may of course use a desensitizer in which case development may 
be conducted in a comparatively bright light which makes it easier to 
determine the appearance of the image. The makers supply in tube 
form a desensitizer for this purpose, or one may use Aurantia (am- 
monia salt) at a concentration of 1 part to 1000 of water. Pina- 


582 PHOTOGRAPHY 


kryptol Green may also be employed but some difficulty is experienced — 
at times in removing the stain of phenosafranine from the film so it is 
better to avoid this agent. 

Reversal of the Image.—I{ we were to fix the image at this stage, 
we would secure a negative image in complementary colors: ‘The 
image secured by development represents exactly the reverse of what 
we require; the silver deposit obstructing the light which should be 
transmitted while that which should be stopped is being transmitted. 
It is necessary, therefore, to reverse the image. This involves (1) the 
removal of the developed silver image and (2) the redevelopment of 
the remaining silver salt to form the positive image. Accordingly as 
soon as development is complete the plate is rinsed in a tray of clear, 
cold water and slipped into the following solution of potassium per- 
manganate which dissolves the silver image: 


Potassium. permanganate, ..< ..2¢)0) sole 30 gr. 2 gm. 
Sulphuric acid 66° 25555 5 08 Gens ee coe ee 3 athe 10 cc. 
Water to make wo. 0.0.0 sien as eee 35 OZ. 1000 cc. 


As soon as the plate is covered with this solution the darkroom may 
be left and all succeeding operations conducted by full daylight, pref- 
erably near a brightly illuminated window. In this solution the image 
rapidly disappears and in 30 or 40 seconds is gone completely. The 
plate is then taken from the solution and carefully washed for about 
half a minute in running water and then replaced in the first developer, 
which should be retained for this purpose. In this the image re- 
appears, this time as a positive, and development is complete within 
four to five minutes. There is no fear of over development, however, 
while complete conversion of the silver salt to the metallic state is es- 
sential to the brilliancy and the permanency of the image. Care should 
be taken, therefore, that development is not stopped too soon. 

After this second development, the plate is washed for three or four 
minutes in running water, taking care that the water does not strike 
the plate with any undue force as the film is very tender at this stage. 
It is then placed on the drying rack and dried as quickly as possible by 
‘means of an electric fan if available. On no account must heat or 
alcohol be used. 

Varnishing.—Although this operation is not absolutely eontial it 
is to be advised since it increases the brilliancy of the colors and serves 
to protect the image from injury. Varnish for this purpose may be — 
secured from the makers of the plates or prepared according to the 4 
following formula: } 


t NATURAL COLOR PHOTOGRAPHY . 583 


Pere EAU GODENZENE aes ki bles See sca dateaaes 100 cc. 5 Oz. 
Re nV fie ne dw ds Axcesa¥e fh ala a 20 cc. oye 


This is flowed over the plate in the usual manner after which the plate 
is placed on the drying rack in a place away from dust where it must 
not be disturbed until completely dry. No varnish containing alcohol 
must be used. 

After-Treatment of Autochromes.—If after development the trans- 
_ parency lacks brilliancy and appears dull and brownish a clearing bath 
may improve matters. For this purpose a 2 per cent solution of 
sodium bisulphite may be employed. 

A general thinness and lack of body in the colors may be due to 
either over exposure or sometimes, but less often, over development. 
Intensification will make some improvement. After-treatment of any 
kind is risky, however, as the film is apt to soften and frill and if 
carried out directly after development a hardening bath of formaline or 
alum should be employed. Another point which requires attention 
when a color plate must be intensified is complete development, other- 
wise the reduction which takes place in the fixing bath will render the 
plate useless. Therefore, if intensification appears to be necessary one 
should make sure that the second development is carried to completion. 

For intensification the makers recommend : 


Be se hc bce ede tee pe sin 3 gm. 45. gf. 
Re gs ee Se cnn utp 3 gm. ae: 0 Br 
Peigcea water 100. i.e. } te deh ig Si ae 1000 cc. ase "oe 

eT TRUE i ied hae bk weeds ould eles 5 gm. We ogy, 
Wy eS os a a i a ae 1000 cc. 34 oz. 


For use take Solution A 10 parts, B 1 part. The chromium intensifier 
may also be used, in fact any method which does not produce a colored 
deposit. | 

With the formula given intensification is quite rapid, from 20-30 
seconds being sufficient in most cases. The solution slowly turns 
yellow and becomes turbid and the plate will then be stained unless 
transferred immediately to a fresh solution. After intensification the 
plate is cleared by immersion for a few seconds in a 0.001 per cent 
solution of neutral potassium permanganate, then after a short wash- 
ing it is placed for two minutes in an acid hypo fixing bath prepared 
as follows: | 


Rl rR er ithe isch sd. cheis- [hs 6-4 dia'a ide aie wo 150 gm. 5% oz. 
Saturated solution sodium bisulphite............ 50 cc 134 Oz 
MNT Oe im, esis og an exces eden a's's 1000 cc. ae Oe, 


Fixing must not be omitted when the plate has been intensified. ) 
g 1 


‘as! aah oe 
) ee ae 


584 PHOTOGRAPHY. is : 


A final wash of four to five minutes completes the process. 

The Agfa Color Plate-——The Agfa color plate, like the Lumiere 
Autochrome, is a mosaic, multi-color, screen plate. The plate itself, as 
well as the operations of producing color transparencies with it, very 
closely resembles the Autochrome. The individual color elements are 
about the same average size as in the Autochrome plate but are more 
uniform, varying in size from 0,008-0.017 mm. The screen as a 
whole, however, transmits very nearly twice as much light as the 
Autochrome; the relative transmissions being 14 per cent for the Agfa — 
plate and 7.5 per cent for the Autochrome. The manipulation of the — 
Agfa color plate differs from that of the Autochrome only in some 
minor details. 

Duplicating Processes of Screen-Plate Color Photography.—De- 
spite the obvious advantages of a duplicating process employing a 
separate taking screen, such methods have not met with commercial 
success. One of the earliest of such plates was the Joly-MacDonough, 
issued about 1892, but discontinued on account of difficulties met with 
in the production of the taking screen-plate. The Thames plate, in- 
troduced several years later, enjoyed a brief spell of popularity and 
was finally replaced by the Paget Duplicating Process which was es- 
sentially an improved Thames plate. This was probably the most 
successful of the separate screen-plate methods but was discontinued 
early in 1925. Soon after the disappearance of the Paget method a 
similar process, but of higher speed, was announced by Chas. Baker 
of High Holborn, London. This is the only representative of sepa- 
rate-screen methods now on the market. 

The Duplex Method.—The exposure is made with the special taking » 
screen in contact with the panchromatic emulsion specially provided for 
the process. After exposure, the plate is developed in the usual 
manner, a desensitizer being employed if desired. Intensification or 
reduction of this negative may be carried out exactly as with other 
negatives. From this negative any required number of transparencies 
may be made on black-tone transparency plates and these positives 
when superimposed in exact register on the viewing screen reproduce | 
the colors of the original subject. 7 

The process thus permits of unlimited duplication without loss of | 
quality since as many transparencies as desired may be made from the 
original negative by the simple operation of contact printing. Besides 
this important advantage there is another no less important: i.e. the 
greater speed of the separate plate method. As shown by Mr. F. E. — 


NATURAL COLOR POT OGRA PEL Y. 585 


Ives the tri-color filters used for making the three color-sensation nega- 
tives should divide the spectrum into approximately three equal parts, 
while the three filters used for viewing purposes should transmit only 
very narrow bands of the three colors. With the combined plate 


Fic. 238. Duplex Screen 


naturally a compromise must be made for one screen must serve both 
purposes, but with separate screens the taking screen can be made 
lighter, thus reducing the exposure required. The Paget process was 
considerably faster than the Autochrome plate and the new Duplex 
method is from four to five times as fast as the latter. With a lens 
having an aperture of F'/4.5 full exposure in bright light will be 
secured at about 44 9 of a second, thus permitting hand camera ex- 
posures under favorable conditions. 

Not all is plain sailing, however, for there are some tran bact to 
the separate screen method. The most important is the parallax error 
arising when the image is not viewed at exactly right angles. When 
examined from any other than a right angle the patch of silver deposit 
in the transparency is not in line with its appropriate color screen but 
the one to the left or right of it. The colors vary, therefore, with the 
angle from which the image is observed and only by looking at it 
perpendicularly can the proper colors be seen. This defect of parallax 
is present to a greater or less degree in all separate plate processes and 


586 ) ePHOTOGRAPE YT 


unfortunately the smaller the color elements the greater is the parallax — 


error, 


Other drawbacks are the difficulty of securing perfect contact be- — 


tween the taking screen and the sensitive plate and between the view- 
ing screen and the positive transparency—a condition which becomes 
increasingly difficult with an increase in the size of the transparency. 
Registration also presents some difficulties at times but these are but 
minor matters which do not radically affect the performance of the 
process. | 


GENERAL REFERENCE WorKS 


AsNnEy—Color Measurement and Mixture. 

BoLas, TALLIENT AND SENIOR—Photography in Colors. 

BrowNn—Color Photography. (Photo-Miniature No. 128.) 

CLERC AND CAMELS—La Reproduction Photographique des Couleurs. 
Ducos pu Havron—La Triplice Photographique des Couleurs. 
Husit—Three-Color Photography. (English Translation by H. O. Klein.) 
Hust—Die Dreifarbenphotographie. 

Jounson—Photography in Colors. , 

Konic—Natural Color Photography. (English Translation by E. J. Wall.) 
KronE—Die Darstellung der naturlichen Farben. 

Mees—Color Photography. (Photo-Miniature No. 183.) 

VALENTA—Die Photographie in naturlichen Farben. 

VipaAL—Photographie des Colours. 

VipaLt—Traite pratique de Photochromie. 

Watit—Practical Color Photography. 

Wati—History of Three-Color Photography. 

Color Photography—lInstructions. (Photo-Miniature No. 147.) 


Per NN DX 


List OF THE PRINCIPAL REFERENCE WORKS ON PHOTOGRAPHY 


In ENGiisH, FRENCH AND GERMAN 


REFERENCES TO TECHNICAL JOURNALS 


APPENDIX 


A LIST OF MORE IMPORTANT REFERENCE WORKS ON 
PHOTOGRAPHY 


Note.—The following list contains the titles of general reference 
works only. For works relating to any particular subject see the 
short bibliographies at the end of each chapter. Works are classified 
according to the language in which originally printed. Translations 
are also listed where published in book form. Works which are now 
out of print have been included where especially valuable. Although 
these can no longer be obtained from the publishers, they may be 
located from time to time by the large dealers in second-hand technical 
works. 


REFERENCE WoRKS IN ENGLISH 


ABNEY—Instruction in Photography, 10905. 

AsNEy—Treatise on Photography, 1903. 

AsnEY—Photography with Emulsions, 1806. 

BayLEy—The Complete Photographer, 1923. 

BrotHers—A Manual of Photography, 1890. 

Dererr—Photography for Students of Physics and Chemistry. 

Jones—Science and Practice of Photography. 

Jones—Photography of Today. 

Jones—Cassell’s Cyclopedia of Photography, 1912. 

MEES AND SHEPPARD—Investigations on the Theory of the Photographic Process, 
1907. . 

MeELpoLAa—The Chemistry of Photography. 

MorTIMER AND Watt—The Dictionary of Photography. 

RorBucK—The Science and Practice of Photography. 

WatTxins—Photography—Its Principles and Applications, 1912. 

Woopspury—Dictionary of Photography, 1897. 

FLrint—The Chemistry of Photography, 1918. - 

The Physical Chemistry of the Photographic Process, 1923. 

Photography as a Scientific Implement, 1923. 


REFERENCE WorkKsS IN GERMAN 


Davip—Lehrbuch der Photographie. 

Davin—Photographisches Praktikum. 

Eper—Ausfiihrliches Handbuch der Photographie, 1885-1903. In four volumes. 
(The most complete and authoritative work on the subject in existence.) 


39 589 


590 PHOTOGRAPHY 


EpER AND VALENTA—Beitrage zur Photochemie, 1904. 
GotpBerc—Der Aufbaudes der Photographischen Bildes, 1920. 
LIESEGANG—Photographische Physik. 
LiesEGANG—Photographische Chemie. 
Lupro-CraMER—Kolloidchemie und Photographie, 1921. 
LuTHER—Die Chemischen Vorgange in der Photographie, 1899. 
LaNIER—Photochemische Chemie und Photochemie, 1899. 
Metruie—Lehrbuch der Praktischen Photographie. 
Pi1zzIGHELLI—Handbuch der Photographie. 
PLoTNIKOW—Photochemische Versuchstechnik. 
PLotNikow—Grundriss der Photochemie, 1923. 
ScHMiIptT—Kompendium der Photographie, 1920. 

ScH Mipt—Photographiren. 

ScHMipt—Vortrage uber Chemie und Chemilalienineae fur Photographierende. 
StoLz—Chemie fur Photographen. 

VALENTA—Photographische Chemie und Chemikalienkunde, 1920. 
WENTzEL—Die Photographisch-chemische Industrie, 1926. 


REFERENCE WorKS IN FRENCH 


BreLtin—Precis de Photographie Generale, 1905. 
Braun—Dictionnaire de Chimie Photographique, 1904. 

CLerc—La Photographie Practique. 

Coustet—Out en est la Photographie. 

DavannE—La Photographie, Traite Theoretique et Practique, 1888. 


Faspre—Traite Encyclopedique de Photographie, 1889-1906. Eight volumes 


(The standard reference work in French.) 
HeEnri—Etudes de Photochemie, 1919. 
MatTHET—Traite Chimie Photographique. 
PouLENc—Les Produits Chemiques en Photographie. 
SEYEWETZ—Le Negatif en Photographie. 


REFERENCES TO TECHNICAL JOURNALS 
Chapter I. The Development of Photography 


(For list of general reference works see page 34) 


CromMER—Deux Details Historiques. Bull. Soc. franc. Phot., 1923, p. 250. 

CroMER—Une Lettre de Nicephore Niepce. Bull. Soc. franc. Phot., 1922, p. 60. 

PoTONNIEE—Baynard and The Invention of Photography. Brit. J. Phot., 1914, 
61, 43. , 

PotonniEE—The Cardinal Plate of Niepce. Brit. J. Phot., 1920, 67, 29. 

PoTtonNniEF—Date of the Invention of Photography. Bull. Soc. franc. Phot., 
F025, 0 312. 

PotonNiEE—The Origin of the Camera Obscura. Bull. Soc. franc. Phot., 1923, 
p. 52. 

TENNANT-Woops—Early Daguerreotypers in the United States. Brit. J. Phot., 
1920, 67, 420. 

WATERHOUSE—History of the Camera Obscura. Phot. J., 1900, 40, 270. 

WATERHOUSE—The Development of Photography with Salts of Silver. Phot. 
J., 1903, 43, 159. 

W ATERHOUSE—Robert Hooke’s Portable Camera Obscura. Phot. J., 1909, 49, 
348. 

WATERHOUSE—Robert Boyle’s Portable Camera Obscura. Phot. J., 1900, 49, 
333- 


Chapter II. The Camera and Darkroom 
THe ARRANGEMENT OF THE DARKROOM 


Brown—Fitting up the Darkroom. B. J. Almanac, 1913, p. 523. 
Davis—The Arrangement of a Darkroom. Amer. Phot., 1913, 7, 108. 
GEAR—Fitting up the Darkroom. Phot. J., 1911, 51, 338. 

Kinc—That Model Darkroom. Amer. Phot., 1920, 14, 67. 
Krart—Shutter for Darkroom Window. Amer. Phot., 1915, 9, 664. 
LaFER—My Darkroom. Amer. Phot., 1913, 7, 579. 

Roserts—The Evolution of a Darkroom. Amer. Phot., 1916, 10, 16, 238. 
WEston—Darkroom Fittings. Phot. J., 1921, 61, 25. 


On Darkroom SAFELIGHTS 


Hartrice—Darkroom Safelights. Brit. J. Phot., 1915, 63, 503. 
HickMAN—Illumination of the Darkroom by Means of Lamps in Liquid Cells. 
Phot. J., 1920, 60, 147. 
Merres—Darkroom Illumination by Reflected Light. Brit. J. Phot., 1915, 62, 603. 
Mees AND BAKER—Measurement of the Efficiency of Darkroom Light Filters. 
Phot. J., 1907, 47, 276. 
NEUGEBAUER—Preparation of Darkroom Safelights. Brit. J. Phot., 1923, 70, 
397- 
591 


592 PHOTOGRAPHY 


PLepGE—Darkroom Illumination. Brit. J. Phot., 1921, 68, 249. 

STENGER—Liquid Darkroom Safelights. Brit. J. Phot. 1905, 52, 732; Zeit. 
wiss. P., 1905, 2, 233. | 

TRIVELLI—Lights for the Darkroom. Brit. J. Phot., 1911, 58, 474, 404, 533, 628, — 
777, 872, 957; 1912, 59, 22. | 


Chapter III. Photographic Optics 
(For list of general reference works see page 84) 
Foca, LenctH AND Its DETERMINATION 


Jostinc AND Satt—Measurement of Focal Length by Clay’s Method. Brit. J. 
Phot., 1922, 69, 137. 

JoHnson—Focal Length of a Lens or Lens Combination. Phot. J., 1906, 46, 
300. 

JOHNSON AND GLEICHEN—Summary of Laws Relating to Focal Length. Phot. 
J., 1913, 53, 183. | 

LAMBERT—Measuring the Focal Length of a Lens. Phot. Journal of America, © 
1923, 60, 87. 

Locxett—A New Method for Finding the Focal Length of Lenses. Brit. J. 
Phot., 1915, 62, 411; Brit. J. Phot., 1922, 69, 434. . 

—.—Measuring Focal Length. (Summary of Methods.) Brit. J. Phot. — 
1916, 63, 79. 

DeptH oF Focus 


BrowN—Theory and Practice of Depth of Focus. Brit. J. Phot., 1922, 69, 
492, 507, 521, 534. 
BrowNnE—A Simple Depth Chart. Brit. J. Phot., 1923, 70, 775. 
Cottins—Depth of Focus and Its Graphical Representation. Brit. J. Phot. — 
1920, 67, 645, 659. 
Fraprig—Table of Hyperfocal Distances. Brit. J. Phot., 1915, 62, 795. 
Jounson—Calculating the Distance Beyond Which Everything is in Focus. — 
Phot. J., 1906, 46, 320. | 
Lee—Chart for Finding the Depth of Focus. Phot. J., 1922, 62, 229; Brit. J. — 
Phot., 1922, 69, 135. 
PipeR—Depth Simplified. Brit. J. Phot., 1905, 52, 1004. 
Prrer—Depth and the Sine Condition. Brit. J. Phot., 1906, 53, 125. 
Pirper—Causes of Variation in Depth of Focus. Brit. J. Phot., 1903, 50, 666, — 
687. 1 
RupotpH—A New Depth Test Object. Phot. Rund., 1921, p. 266. 


SCALE OF OptTIcAL REPRODUCTION 
BrowN—Scale of Optical Reproduction. Brit. J. Phot., 1921, 68, 667, 685, 702. 


Loss or Licut 1n Lens SysteMs By ABSORPTION AND REFLECTION 


CuHESHIRE—The Loss of Light in Lenses. Brit. J. Phot., 1912, 59, 507, 645. : 
Morritr—The Light Absorbed by Lenses. Phot. Journal of America, 1920, 59, q 


AII. - 
Nuttinc—The Brightness of Optical Images. Phot. J., 1914, 54, 187. : 


BEPERENCES TO -TECHNICAL JOURNALS | ‘593 


OpENcRANTS—The Experimental Determination of the Luminosity of Photo- 
graphic Objectives. Nord. Tids. Fot., 1925, 9, 21; S. I. P., 1925, 5, 87. 

ZscHOKKE—Factors Other than Aperture in the Rapidity of a Lens. Brit. J. 
Phot., 1912, 59, 823. 


Chapter IV. The Aberrations of the Photographic Objective 
(For list of general reference works see page 103) 


ON THE ABERRATIONS OF PHOTOGRAPHIC OBJECTIVES AND THE TESTING OF 
OBJECTIVES 


Bennett—Aberrations of Long Focus Anastigmatic Objectives. Bur. Stand- 
ards Paper, No. 404. _ 

Bow—On Photographic Distortion. Brit. J. Phot., 1861, 8, 417. 

Bow—On the Curvature of the Image. Brit. J. Phot., 1863, 10, 228. 

Bow—On the Loss of Light from Obliquity of Incidence. Brit. J. Phot., 1866, 

. 13, 159. 

Carson—The Correction of the Aberrations of a Photographic Objective. Phot. 
J., 1903, 43, 188, 278. 

CHALMERS—The Aberrations of Photographic Objectives. Phot J., 1007, 47, 
374- 

CLay—Determining the Focal Length and Aberrations of a Photographic Ob- 
jective. Phot. J., 1904, 44, 180. 
Grusps—On the Equalization of the Photographic Image in Fields of Large 
Angle Projected upon a Flat Surface. Brit. J. Phot., 1863, 10, 4or. 
Gruss—Depth of Focus and Spherical Aberration. Brit. J. Phot., 1867, 12, 61. 
Houpaitte—Sur D’essai Scientifique et Pratique des Objectifs Photographiques. 
Bull. Soc. franc Phot., 1893, 9 (2 Series), 257. Librarie Gauthier-Villars, 
1893. 

KoHLrANscH—Testing Photographic Objectives. Phot. Korr., 1920, p. 45. 

KoLtLMorcEN—Achromatic and Apochromatic Correction. Phot. J., 1902, 42, 180. 

LenouveL—Methode de Determination et de Mesure des Aberrations des Sys- 
tems Optiques. S. T. I. P., 1924, 4, 33. 

Morssarp—Appareil pour L’Etude Experimentale Complete des Lentilles et des 
Objectifs Photographiques. Bull. Soc. franc Phot., 1889, 5 (2 Series), 
124. 

RHEDEN—Reflections in Lenses. Phot. Rund., 1921, 101. 

“Rooms ”—Lens Corrections. Phot. J., 1907, 47: Part I, p. 24; Part I, 
py, eae; Partiiil, p,.279; Part IV, p..330; Part V, p, 351. 

“ Rooms ”—Achromatism. Phot. J., 1908, 48: Part I, p. 320; Part II, p. 333; 
Pare), 344. Part 1V, p. 375; Phot: J., 1000, 49: Part V, p. 54; Part 
VI, p. 126. 

“ RHoms ”’—Astigmatism. Phot. J., 1909, 49, 417. 

“Ryoms”—An Exact Formula for Spherical Aberration. Phot. J., 1900, 49, 
381. | 

STEINHEIL—Das Prufen und Wahlen der Photographen-Objectiv. Phot. Korr., 
1869, 6, 49. 

Taytor—Axial Aberrations of Lenses. Brit. J. Phot., 1918, 65, 101, 113, 124. 

TayLor—Lens Testing Instruments. Phot. J., 1902, 42, 40. 


594 PHOTOGRAPHY 


TILLYER AND Kerr—Lens Testing Instrument. U. S. P., 1, 383, 578. 

TwyMANn—On the Use of the Interferometer for ‘Tevine Photographic Ob- J 
jectives. Phot. J., 1919, 59, 239. fl 

TuHompson—Zonal Aberration and its Consequences. Phot. J., 1900-01, 40, sau 
British Journal Almanac, 1902. 


Chapter V. The Photographic Objective 


(For list of general reference works see page 146) 
PAPERS ON THE DEVELOPMENT OF THE OBJECTIVE 


BuNnGer—Genesis of Modern Lenses. Brit. J. Phot., 1907, 54, 638, 660, 736. 

CLray—The Photographic Lens from a Historical Point of View. Phot. J., 
1922, 62, 459. 

DaLLMEYER—The Evolution of Modern Lenses. Phot. J., 1900-01, 40, 64. 

LUMMER—Beitrage zur photographischen Optik. Zeitsch. Instrument, 17, 208, 
225, 264. 

Von Rour—Uber die Bedingungen fur die Verzeichnungsfreiheit optischer oe 
teme mit besonderer Bezungnahme auf die bestehenden typen Photo- 
graphischer. Zeitsch. Instrument, 1898, 17, 271. 

Von Rour—Beitrage zur Kenntniss der geschichten Entwicklung der Ansichten 
uber die Verzeichnungsfreiheit photographischer Objectiv. Zeitsch. In- 
strument, 1898, 18, 4 i 

Von Rour—Uber die Lichtvertheilung in der Brennebene photographischer Ob- 
jectiv mit besonderer Berucksichtigung der bei einfachen Landschaftslinsen 
und symmetrischen Konstruction auftrenden Unterschiede. Zeitsch. In- 
strument, 1898, 18, 171, 197. 


Von Rour—Die Entwicklungeschichte der getrauchlichen Typen Photograph- — 


ischen Objectiv. Eder’s Jahrb., 1900, 14, 106. 


Von Rour—Development of Symmetrical Objectives with Central Diaphragm, 


Composed of Equal or Similar Halves up to the Time of the Aplanat. 
Central Zeitschrift Mech. Optik., 1921, p. 327. E 
Von Rour—Contributions to the History of the Photographic Objective in — 
England and America between 1800-1875. Phot. J., 1924, 64, 3590. — ; 


Papers RELATING TO INDIVIDUAL LENSES 


Ax.pis—Astigmatism and a New Stigmatic Lens. Phot. J., 1895, 35, 117; Brit. — 
J. Phot., 1896, 43, 262, 280. 

Beck—A New Principle in Photographic Lens Construction. Phot. J., 1 
44, 172. 

Brck—The Isostigmar. Phot. J., 1907, 47, 191. a 

Hartinc—Recently Discovered Objectives of Petzval and Zinc-Sommer. Phot. — 
Ind., 1924, p. 1030. . 

KiuGHarpt—The Ernemann Ernostar F/2 Anastigmat. Phot. Ind., 1924, p 
1008. a 

Lre—The Taylor, Taylor and Hobson F/2 Anastigmat. Trans. Opt. Soc., 1924, — 
25, 240. i. 

MertE—The Tele-tessar. Cent. Zeit. Mech. Opt., 1921, p. 245. 

MieTtHE—Symmetrisches Objectiv ohne Astigmasie. Phot. Mitt., 1888, 25, 123. 


REFERENCES TO TECHNICAL JOURNALS 595 


Puyo anp PutticNy—The Anachromats. Brit. J. Phot., 1906, 53, 184. 

RupoLpH—Anastigmatic Aplanatism and the Zeiss Lenses. Brit. J. Phot., 1803, 
40, 481. 

RopENSTOCK—Bistigmatsatz. Phot. Rund., 1901, —, 37. 

Von Rour—Uber das Planar. Eder’s Jahrbuch, 1898, 12, 70. 

Von Rour—Uber altere Portratobjektive. Zeitsch. Instrument, 1901, 21, 40. 

WenuHAM—Achromatic Periscope. Brit. J. Phot., 1874, 21, 507, 621; Brit. J. 
Phot., 1875, 22, 22. 


PATENTS ON LENSES 


(B. P., British Patent; D. R. P., German Patent; U. S. P., United States 
Patent; B. F., French Patent) 


ABBE AND RupotpH—Photographic Triplet. D.-R. P. 55,313/1890, B. P. 6020/00. 

A.pis—The Stigmatic. B. P. 16,640/95, D. R. P. 92,582/95. 

A.tpis—The Aldis Triplet. B. P. 5170/02. 

ARBEIT—Symmetrical Anastigmatic Objective. D. R. P. 135,742/o1, D. R. P. 
ery a . . 16,431/11. 

Beck—Isostigmar. B. P. 27,180/1906, B. P. 14,673/1908, D. R. P. 104,267. 

Beck—Neostigmar. B. P. 2619/1911, B. P. 3399/1911, B. P. 4714/tTo11. 

BoorH—Pentac. B. P. 151,506/20. 

CLrarK—Objective. U.S. P. 399,400/1880. 

DALLMEYER—Rapid Rectilinear and Modified Petzval Lens. B. P. 2502/1866. 

DALLMEYER—Rectilinear Landscape Lens. B. P. 1853/1888. 

DALLMEYER—Single Landscape Lens. B. P. 2530/1864. 

DALLMEYER—The Achromatic Triplet. B. P. 3096/1866. 

Gorrz—Cf. Von Hoegh and Zschokke. 5 

Grar—Graf Anastigmat. U. S. P. 1,463;132/1923, B. P. 22,400/1o910, U. S. P. 
981,412/II. 

Gruss—Grubb’s Lens. B. P. 1968/1871. 

GunpLacH—Turner-Reich Anastigmat. 

Harrison—The Globe Lens. B. P. 2496/1860. 

KaAEMpFER—Kollinear. D. R. P. 00,482/1895, D. R. P. 91,883/1805. 

KoLttMorcen—Aristostigmat. D. R. P. 125,560. 

Lan—Davis-Serrac. B. P. 27,518. 

Lacour BertHiotr—Anastigmat. B. F. 374,045/1907. 

Lacour Bertuiot—Stellor. B. F. 456,484/1913. 

Lee—Unsymmetrical Anastigmatic Objective. B. P. 200,371. 

Lertrz—Unsymmetrical Anastigmatic Objective. D. R. P. 116,440. 

Martin—Omnar. O. P. 8364/1901. 

Morrtson—Wide Angle Lens. U. S. P. 126,970. 

Potak—Hyperchromatic Objectif. B. P. 201,920. 

Rupo_tPH—Doppel-Anastigmat. B. P. 4692/1803, B. P. 19,500/1804. 

Rupotra—tThe Planar. D. R. P. 92,313, B. P. 27,635/1806. © 

RupotpH—Unar. D, R. P. 134,408/1899, B. P. 24,080/1809. 

Rupo.pH—Tessar. D. R. P. 142,294/1902. 

RupotpH—Protar Series VIIa (New Construction). D. R. P. 228,667/19009, 
B. P. 23,604/1909. 

RupotpH—Plasmat. B. P. 161,091/1920, 


596 PHOTOGRAPHY 


REICHERT—Solar. D. R. P. 180,255/1904. 

REICHERT—Combinare. D. R. P. 153,525. 

ReirzscHEL—Linear. D. R. P. 118,466/1808. 

RopENstockK—Imagonal. D. R. P. 177,266. 

ScHROEDER—Concentric Lens. B. P. 5194/88. 

ScHROEDER—Achromatic Periscope. U.S. P. 554,737/06. 

SmitH—Air Space Doublet. B. P. 133,459. 

STEINHEIL—Orthostigmat. D. R. P. 76,662/93. Orthostigmat Type II. D. R. P. 
88,505/03. 

STEINHEIL—Antiplanet. D. R. P. 16,354/81, B. P. 1602/81. 

STEINHEIL—Unofocal. D. R. P. 133,957. 

STEINHEIL—Portrait Aplanat. B. P. 1124/74. 

STEINHEIL—Group Aplanat. D. R. P. 6189/79. 

STEINHEIL—Periskop. B. P. 2037/65. 

Simon—Octanare. D. R. P. 168,977. 

Taytor, H. D.—Cooke Triplet. B. P. 22,607/93, D. R. P. 81,825/o04, B. P. 
15,107/95, D. R. P. 86,757/05, B. P. 24,391/1905, B. P. 3398/1905, B. P. 
7661/1906, B. P. 3799/1912. 

Von HorcH—Dagor. D. R. P. 74,437/92, B. P. 23,378/o92. 

Von HoecH—Improved Form of the Dagor. B. P. 13,162/95. 

Von Horcuo—The Celor and Syntor. D. R. P. 1009,283/08, B. F. 278,768/08, 
B. F. 320,304/03, D. R. P. 202,083/07. 

VoIGTLANDER—Modification of the Petzval Objective. D. R. P. 5761/78, B. P. 
4756/78. 

VoIGTLANDER—Euryscope. D. R. P. 5761/78, B. P. 1938/77. 

ZeIss—Triplet Anastigmat. D. R. P. 86,757/95, B. P. 6328/13, B. F. 455,546/13. 

‘Ze1ss—Four Lens Symmetrical Anastigmat. U. S. P. 1,479,197/23. 

ZsCHOKKE—Dogmar. D. R. P. 258,495/12, B. P. 833/13, B. F. 453,230/13. 


THeE TELEOBJECTIVE 
Reference Works 


DALLMEYER—Le teleobjectif et la photographie. (French translation by L. P. 
Clerc. 1904. English edition out of print.) 

Lan Davis—Telephotography. 

ScuHmMipt—Das Teleobjectiv. 

Von Rour—Zur Geschichte und Theorie des Photographischen Teleobjektivs 
mit besonderer Berucksichtigung der durch die seiner Strahlen Begrenzung 
bedingen Perspectiv. 1897. 

W HEELER—T elephotography. 


Papers 
DALLMEYER—The Adon and Notes on Telephotographic Systems. Phot. J., 1902, 
42, 97. 
DALLMEYER—A New Teleobjective for Photography. J. Cam. Club (London), 
1892, p. 10, Eder’s “ Photographischen Objektiv,” p. 171. : 


DaLLMEYER—The Compound Telephoto Lens. J. Cam. Club (London), 1802, 
Eder’s “ Photographischen Objektiv,” p. 175. 

Lre—Principles and Construction of the Telephotographic Lens. Phot. J., 1925, 
65, 392. 


REFERENCES TO TECHNICAL JOURNALS 597 


MretHe—Fin neues telephotographisches System. Eder’s Jahrb., 1892, 6, 152. 
Von Rour—Petzval’s Orthoskop. Eder’s Jahrb., 1900, 14, 108. 
Von Ronr—Zur Entwicklungsgeschichte des Teleobjektivs und seiner Theorie. 
Eder’s Jahrb., 1897, 11, 181. 
WaterHOUSE—Lens Systems and the Genesis of Telephotography. Phot. J., 
1902, 42, 4. 
PATENTS ON TELEOBJECTIVES 


ae AND STUART—The Telecentric. B..P.. 2349/12. 

Bootu—The Dallon. B. P. 151,506/19, B. P. 151,507/19, U. S. P. Spokane 

Booth—The Telic. B. P. 1,156,743/1915. 

Lan Davis—The Large Adon. B. P. 1185/14. 

LrE—Teleobjective. B. P. 132,067/1018. 

Martin—The Busch Bis-Telar. B. P. 15,732. 

Merte—The Tele-tessar. B. P. 145,548/19, B. P. 170,520/21, U. S. P. 1,467,- 
804/23. 

STUART AND BreLtickKE—The Teleros. B. P. 188,621/22. 

W HEELER—Telephotographic Lens. B. P. 18,121. 

ZE1ss—Teleobjectives. B. F. 363,499/06, B. P. 4532/06, D. R. P. 227,112/08, 
B. F. 11,701/09, B. P. 19,580/09. 


Chapter VI. The Photographic Emulsion 


(For list of general reference works see page 171) 


GELATINE IN PHOTOGRAPHY 


FinpLAy—Some Properties of .Colloidal Matter and Their Applications in 
Photography. Phot. J., 1920, 60, 223. . 
SHEPPARD—Colloid Chemistry and Its Relation to Photography. Phot. J., 1900, 

49, 320. 

SHEPPARD—The Modern Chemistry of Gelatine. Brit. J. Phot., 1922, 69, 677, 
695, 706, 710; J. Ind. and Eng. Chem., 1922, 14, 1023. 

SHEPPARD, ELLIOTT AND SwEET—Photographic Properties of Gelatine. Physical 
Chemistry of the Photographic Process. (Published as Transactions of 
the Faraday Society.) 

SHEPPARD—Photographic Gelatine. Phot. J., 1925, 65, 380. . 

StapE—Colloid Chemistry in Photography. (Bibliography.) Brit. J. Phot., 
1920, 67, 645. . 

SLATER-Price—Gelatine. Phot. J., 1922, 62, 356. 

SLATER-Price—Certain Fundamental Problems in Photography. J. Roy. Soc. 
Arts, 1924, 72, 725, 739, 753- 

ScHAUuM—Photographic Binding Media and Other Gels. Koll.-Zeits., 1925, 36, 
199. (Zsigmondy-Festschr.) 


THEORY OF EMULSION PROCESSES 


Bancrortr—The Photographic Plate. J. Phys. Chem., 1910, 14, 12, 97, 201, 620. 
Ex_spen—On the Formation of a Chemical Compound of Ammonia with Silver 
Bromide. Phot. News, 1881, 25, 174. 


598 PHOTOGRAPHY 


GarpicKE—Ammoniakraucherung bei Trockenplatten. Eder’s Jahrbuch, 1913, 
27, 62. 

JarMAN—Photographic Emulsions. Photominiature No. 179. 

JoHNson—Gelatino-Bromide of Silver Emulsions Treated with Ammonia. B. 
J. Almanac, 1877, p. 95. ie 

KnocHEe—Researches on Photographic Ripening. Phot. Ind., 1924, p. 1169. — 

LizeseEGANG—Uber die Reifung von Silberhaloidemulsion. Zeit. Phys. Chests 
IQI0, Pp. 75, 374- 

LirsEGANG—Ripening of Silver Halide Emulsions. Zeit. wiss. Phot., 1923, 22, 
81. 

LizeseEGANG—Intermediate Stages in Emulsion Making. Phot. Ind., 1925, p. 111. 

LoreNz—Kolloidchemie und Photographie. Koll.-Zeits., 1918, 22, 103. 

Luppo-CRAMER—The Ripening Process. Zeit. wiss. Phot., 1924, 23, 84, III; 
1925, 23, 137. ; 

Luppo-CrRAMER—Latent Fog in Emulsions. Z. Angew. Chem., 1924, 37, 40. 

MitryaTtaA—Manufacture of the Photographic Dry Plate. J. Chem. Ind. Japan, 
1921, 24, 884, 906. 

Papyrus—The Ripening of Washed Emulsions. Influence of Foreign Sub- 
stances. Phot. Ind., 1925, p. 372. 

QuincKkE—Die Bedeutung der Oberflachenspannung fur die Photographie mit 
Bromsilbergelatine und eine Theorie des Reifungsprozesses der Bromsil- 
bergelatin. Eder’s Jahrbuch, 1905, 19, 3 

REINDERS—Studien uber die Photohaloide. Zeits. Physik. Chem., 1911, 77, 213, 
357; 677. 

ReNwick—The Manufacture of Sensitive Emulsions as a sti and an Art. 
J. Soc. Chem. Ind., 1923, 42, 43. 

RENwicK—Note on the Factors Affecting Grain Size in Bhoweeiitle Emul- 
sions. Phot. J., 1924, 64, 324. 

ScHARLOW—Preparation of a Silver Bromide Emulsion for Diapositive Plates 
and Bromide Papers. Phot. Ind., 1924, p. 233. 

STEIGMANN—Remarks on Ripening. Phot. Ind., 1925, p. 88. 

TRIVELLI—Beitrage zu einer Theorie des Reifungsprozesses der Silberhaloide. 
Zeit. wiss. Phot., 1910, 8, 17. 


TriveLLI—Influence of Silver Iodide on the Sensitivity of Silver Bromide. 1 Ph 


Soc. Chem. Ind., 1923, 42, 908. 


GRAIN SENSITIVITY 


BrooksBANK—The Darkening of Silver Bromide Grains on Exposure to Light 
as Further Evidence of their Heterogeneity in Photographic Emulsions. 
Phot. J., 1921, 61, 421. 


CrarK—Grain Structure vs. Light Quanta in the Theory of Development. 


Brit. J. Phot., 1922, 69, 462. 

CLrarK—The medgeune Centers of a Silver Bromide Emulsion. Phot. J., 1923, 
63, 230. Brit.-J. Phot., 1923, 70, 227. 

CLaRK—Sodium Arsenite aod the Plate. Brit. J. Phot., 1923, 70, 717. 

CLark—On the Sensitivity of the Silver Haloid Grains of a Photographic Emul- 
sion. Phot. J., 1924, 64, 91. 3 


7 
; 
; 

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: 
: 
: 


REFERENCES TO TECHNICAL JOURNALS 599 


CirarK—On the Sensitivity of a Silver Bromide Emulsion. The Physical Chem- 
istry of the Photographic Process. (Published by the F araday Society.) 
CLrarK—Reversal by Hydrogen Peroxide, Sodium Arsenite and Light. Phot. 


J., 1924, 64, 363. 
CLarK—The Action of Arsenites on the Photographic Plate. Brit. J. Phot., 
1925, 72, 155. 7 | 
Renwick—The Sensitive Centers of Silver Bromide Grains. Brit. J. Phot., 

1923, 70, 359. 


SILBERSTEIN—Quantum Theory of Photographic Exposure. I. Phil. Mag., 1922, 
44, 257; Il. Ibid., 1922, 44, 956; III. Ibid., 1923, 45, 1062. 

SHEPPARD, WIGHTMAN AND TRIVELLI—Exposure Theories. The Physical Chem- 
istry of the Photographic Process. (Published by the Faraday Society.) 

SHEPPARD AND TrRivetLI—Influence of Crystal Habit on the Photochemical De- 
composition in Silver Bromide Crystals. Phot. J., 1923, 63, 334. 

SHEPPARD AND WiGHTMAN—Note on the Theory of Photographic Sensitivity. 
Science, 1923, 58, 80. 

SHEPPARD AND TRIVELLI—Structure of the Photographic Emulsion. Trans. 
Faraday Soc., 1923, Ig, 270. 

SHEPPARD, TRIVELLI AND WiGHTMAN—Relationship of Photographic Emulsion 
Fog to Grain Size. Phot. J., 1925, 65, 134. 

SHEPPARD, WIGHTMAN AND TrivELLI—The Action of Hydrogen Peroxide on 
Photogranhic Gelatino-Silver Halide Emulsions. J. Frank. Inst., 1923, 
195, 337. 

SHEPPARD, WIGHTMAN AND TRiveLLI—Action of Hydrogen Peroxide on Plates 
with a Single Layer of Grains. S. I. P., 1925, 5, 50. 

SLADE AND Hicson—Photochemical Investigation of the Photographic Plate. 
Proc. Roy. Soc., 1920, 98, 154. 

SLADE AND Hicson—Action of Light on the Photographic Plate. Phot: J., 1921, 
61, 35, 144, 252. 

SLATER-Price—The Modern Conception of the Sensitivity of Photographic 
Emulsions. Phot. J., 1925, 65, 208. 

SVEDBERG—Size and Sensitiveness of the Grains in a Photographic Emulsion. 
Zeit. wiss. Phot., 1920, 20, 36. 

SvEDBERG—The Se ticibility of the Individual Halide Grains in a Biciasehie 
Emulsion. Phot. J., 1922, 62, 183. 

SvEDBERG—On the Relation between Sexsitiveness and Size of Grain in Photo- 
graphic Emulsions. Phot. J., 1922, 62, 186. 

SvepBERG—The Interpretation of Light Sensitivity in Piseenoly: Phot. J., 
1922, 62, 310. 

SVEDBERG, SCHUNK AND ANDERSSON—Relation between Exposure and the Num- 
ber of Developable Centers. Phot. J., 1924, 64, 272. 

Toyv—The Theory of the Characteristic Curve of a Photographic Emulsion. 
Phil. Mag., 1922, 44, 352; II, 1923, 45, 715. 


_ Toy—The Quantum Theory of Photographic Exposure. Brit. J. Phot., 1922, 


69, 443. 

Toy—The Mechanism of Latent Image Formation. The Physical Chemistry 
of the Photographic Process. (Published as Transactions of the Faraday 
Society. ) 


600 | PHOTOGRAPHY 


Toy AND Epcerton—The Relation between the Light Frequency and the Num- q 
ber of Developable Centers. Phil. Mag., 1924, 48, 947. 


SIZE-FREQUENCY DISTRIBUTION 


GERMANN AND HyLan—Dispersity of Silver Halides in Relation to their Photo- 
graphic Behavior. Science, 1923, p. 332. Second Paper, J. Phys. Chem., 
1924, 28, 449. . 

Hicson—The Emulsion for a Process Plate. Phot. J., 1919, 59, 260. 

SHEPPARD, WIGHTMAN AND TRIVELLI—Size-Frequency Distribution of Particles 
of Silver Halide in Photographic Emulsions and its Relation to Sensi- 
tometric Characteristics. I..J. Phys. Chem., 1921, 25, 181; II. Ibid., 1921, 
25, 501; III. Ibid., 1923, 27, 1; IV. Ibid., 1923, 27, 141; V. Ibid., 1923, 27, 
440. 

SHEPPARD—Dispersity of the Silver Halides in Relation to their Photographic 
Properties. First Colloid Symposium Monograph, 1923. 

SHEPPARD—Grain-Size and Distribution in Emulsions. Phot. J., 1925, 65, 31. 

SVEDBERG—Size and Sensitiveness of the Grains of a Photographic Emulsion. 
Zeit. wiss. Phot., 1920, 20, 36. 

WIGHTMAN, TRIVELLI AND SHEPPARD—Photographic Densities Derived from 
Size-Frequency Data. J. Phys. Chem., 1924, 28, 520. 


Chapter VII. Orthochromatics 


(For list of general reference works see page 199) 


Cotor SENSITIZING OF GELATINE EMULSIONS 


ADAMS AND HALLER—The Kryptocyanines—A New Series of Photosensitizing 
Dyes. J. Amer. Chem. Soc., 1920, 42, 2661. 

ApripAt—Sur la Preparation des Colorants Sensibilisateurs dans les Emulsions 
Photographiques. Bull. Soc. Franc. Phot., 1923, p. 283. 

BrAUNHOLTZ—A Comparison of Three Isometric Cyanines J. Chem. se (Lon- — 
don), 1922, 121, 160. : 

CAPSTAFF AND BoutoentPradticten of Panchromatic Sensitiveness welts 
Dyes. Brit. J. Phot., 1920, 67, 7109. . 

MEES AND GUTEKUNST—Some Sensitizers for Deep Red. Brit. J. Phot., 1922, 
69, 474. 

Mitts AND Pope—The Isocyanine Dyes as Sensitizers. Phot. J., 1920, 60, 183. 

Mitts AnD Pope—The Carbocyanines as Photographic Sensitizers. Phot. J., 
1920, 60, 253. | 

Mitts AND BraunHoLZ—The Thioisocyanines. J. Chem. Soc. (London), 1922, 
I2I, 2004. 

Mitts AND PorpE—2-p-Dimethylaminostyrylpyridine Methiodide—A ‘New Photo- 
graphic Sensitizer. J. Chem. Soc., 1922, 121, 946. | 

Mitts AND HamMEeR—The Cyanine Dyes. III. J. Chen Soc. (London), 1920, 
II7, 1550. a 

Mirts—The Cyanine Dyes. IV. J. Chem. Soc. (London), 1922, 121, 455. 


REPERENCES TO TECHNICAL JOURNALS 601 


Mitts AnD BRAUNHOLTZ—The Cyanine Dyes. V. J. Chem. Soc., 1922, 121, 


1480. 

Mrs AND BrAUNHOoLTZ—The Cyanine Dyes. VI. J. Chem. Soc., 1922, 121, 
2004. | 

Mitts AND BrAUNHOLTZ—The Cyanine Dyes. VII. J. Chem. Soc., 1922, rar, 
2804. 


MonpiLLarpD—Mixed Sensitizers. La Procede, 1906, February; Brit. J. Phot., 
1906, 53, 245. 

Newton—Ortho Plates and Sensitizers. Phot. J., 1903, 43, 262; 1005, 45, 15; 
1906, 46, II0, 300. 

Eper—Uber farbenempfindliche Platten zur Spektrumphotographie im Infrarot, 
Rot, Gelb, und Grun. (Pinacyanol blue, Pinachrome blue, Pinachrome 
violet, and Dicyanine A.) Phot. Korr., 1915, 51, 23; Brit. J. Phot., 1917, 
64 (color supplement), 8. 

HAmMER—The Optical and Sensitizing Properties of Some Isometric Isocyanines. 
Phot. J., 1922, 62, 8. 

Hamer—The 6.6’-diacetylamino-1.1’-diethylcarbocyanine. J. Chem. Soc. (Lon- 
don), 1923, 123, 2333. : 

HamMeER—Derivatives of the Methylenediaminaldines. J. Chem. Soc., 1923, 123, 
246. 

Husr—Absorption and Sensitizing Spectrum of the Cyanines. Phot. J., 1906 
46, 133; Das Atelier, 1906, 6, 14. 

Husnix—Color Sensitizing in Theory and Practice. Phot. J., 1900-01, 40, 364. 

Konic—Sensitizing Plates by Bathing. Phot. J., 1905, 45, 370 (Abstract). 

Konic—Pinaflavol—A New Green Sensitizer. P. Rund., 1921, 6, 80; Brit. J. 
Phot., 1921 (color supplement), p. 16; P. Rund., 1921, 6, 193. 

Konic—The Quinocyanines—The Constitution of Pinacyanol. Ber. Deuts. chem. 
Ges., 1922, 55, 3203. 

Louse—Die Wirkung der Farben auf Bromsilbergelatineplatten. Eder’s Jahrb., 
1894, 8, 271. 

MEES AND SHEPPARD—Estimating the Color Sensitiveness of Plates. Phot. J., 
1906, 46, I10. 

MEEs AND WRATTEN—Dicyanine and Photography of the Infra Red. Phot. J., 
1908, 48, 25. 
Newton—The Properties 6f Homocol as a Sensitizer. Phot. J., 1905, 45, 226, 

264. ‘ 

RENWICK AND BLrocH—Auramine as a Sensitizer. Phot. J., 1920, 60, 145. 

ReNwick—The Action of Soluble Iodides on Photographic Plates. Phot. J., 
1921, 61, 34. 

SHEPPARD—The Optical and Sensitizing Properties of the chat int Phot. 
J., 1908, 48, 300. 

SHEPPARD—The Action of Soluble Iodides and Cyanides on the Photographic 
Emulsion. Phot. J., 1922, 62, 88. 

VALENTA—Dyes for Color Sensitizing. Phot. J., 1905, 45, 370, 341. 

Watt—A Review of Recent Work in Color Sensitizing. Brit. J. Phot., 1907, 
54, 365, 386, 406, 464. 


602 PHOTOGRAPHY 


WALTERS AND Davis—Color Sensitive Photographic Plates and Methods of 4 


Sensitizing by Bathing. Bulletin of the Bureau of Standards No. 422; 

J. Frank. Inst., 1922, 193, 103; Brit. J. Phot., 1922, 69, 416, 430. 
WATERHOUSE—Experiences with Red Sensitizers. Phot. J., 1904, 44, 165. 
WRATTEN AND Mees—Wedge Spectrographs. Brit. J. Phot., 1907, 54, 304. 


THE PuHotocRAPHIC LIGHT FILTER 


Davis—A New Method of Measuring the Factors of Light Filters. Phot. J., 
1921, 61, 160. 

Hnatek—Light Filter Formule. Zeit. wiss. Phot., 1915, 13, 133, 271; Brit. J. 
Phot., 1921, 68, 95. 

HopcmMan—Light Filter Formule. Brit. J. Phot., 1922, 69, 6. 

MerEes—Luminosity Filters. Brit. J. Phot., 1906, 53, 430. 

PoTapENKo—Theory and Technique of Light Filters. (Appended to this paper 
is a very complete bibliography of 115 papers on light filters in various 
photographic and. other technical journals.) Brit. J. Phot., 1921, 68, 507, 
522, 534- 

Renwick—Color Values in Monochrome and a New Viewing Filter to Assist 
in Obtaining Them. Phot. J., 1910, 59, 158. 

SmitH—Light Filter Making. Brit. J. Phot., 1921, 68, 459.. 


Chapter VIII. The Latent Photographic Image 


(For list of general reference works see page 225) 


REACTIONS OF THE SILVER HALIDES ON PROLONGED ExposuRE 


Eccert AND Noppak—Silver Halide Emulsions and the Law of Photo-chemical 
Equivalence. Z. Physik., 1925, 31, 922. 

Eccert AND NoppAak—The Photo-chemistry of Silver Compounds. Z. Physik., 
1925, 31, 942. 

Garrison—Influence of Light on the Photo-magnetic Properties of the Silver 
Halides. J. Amer. Chem. Soc., 1925, 47, 622. 

Guntz—The Action of Light on Silver Chloride. Phot. J., 1905, 45, 131. 

Hartunc—The Action of Light on Silver Bromide. J. Chem. Soc. (London), 
1922, p. 682. 

Hartunc—The Photo-chemical Decomposition of Silver Bromide. J. Chem, 
Soc. (London), 1924, p. 2198. 

KocH AND SCHRADER—The Action of Light on Silver Chloride, Silver Bromide 
and Silver Iodide. Z. Physik., 1921, 28, 127. 

Kocu AND Kretss—Change of Mass of Silver Halides on Intense Illumination. 
Z. Physik., 1925, 32, 384. 


ScHWwarz AND Stock—Photo-chemical Decomposition of Silver Bromide. Z. q 


anorg. Chem., 1923, 132, 380. 


ScHWARZ AND Gross—Photo-chemical Decomposition of Silver Chloride. Z. 4 


. anorg. Chem., 1924, 133, 380. 
Stock—The Photo-chemical Decomposition of Silver Bromide. Zeit. wiss. 


Phot., 1925, 24, 132. 


ee a ee ee 


REFERENCES TO TECHNICAL JOURNALS 603 


WEIGERT—Photo-chemistry of Silver Compounds. Sitzungsber. preuss. Akad. 
Wiss., IQ2I, p. 641. 


Quantity oF LicgHT REQUIRED TO Propuce DEVELOPABILITY 


Hetmick—On the Quantity of Light Required to Render Developable a Grain 
of Silver Bromide. J. Opt. Soc. Amer., 1922, 5, 908. : 
HrLtmMick—The Average Quantity of Ultra Violet Energy Required to Render 

Developable a Grain of Silver Bromide. J. Opt. Soc. Amer., 1924, 9, 521. 
SHEPPARD AND WIGHTMAN—Energy Exchanges in the Formation of the Latent 
Image. J. Opt. Soc. Amer., 1922, 5, 913. 


ACTION OF OxipIzING AGENTS ON THE LATENT IMAGE 


Citark—Sodium Arsenite and the Plate. Brit. J. Phot., 1923, 70, 717. 

CLiarK—Action of Arsenites on the Photographic Plate. Brit. J. Phot., 1925, 
72, 155. 

Lupro-CRAMER—Fog by Arsenite. Phot. Ind., 1923, p. 456. 

Lupro-CrAMER—Latent Image Reactions. Phot. Ind., 1924, p. 1007. 

RusseL—Action of Resin and Allied Substances on the Photographic Plate. 
Phot. J., 1890, 39, 345. 

SHEPPARD AND Merges—Action of Substances on the Latent Image. Phot. J., 
1907, 47, 65; Brit. J. Phot., 1907, 54, 33. 

SHEPPARD, WIGHTMAN AND TRIVELLI—Fffect of Oxidizers on the Sensitivity and 
on the Latent Image. J. Frank. Inst., 1924, 198, 507. 

SHEPPARD, WIGHTMAN AND TRIVELLI—Action of Arsenite and Oxidizing Agents. 
J. Frank. Inst., 1924, 198, 629. 


- Sterry—The Action of Oxidizers on the Latent Image. Phot. J., 1907, 47, 170; 


Brit. J. Phot., 1907, 54, 166, 171, 206; Eder’s Jahrbuch, 1907, 21, 364. 


RETROGRESSION 


BAEKLAND—Photo-retrogression. Zeit. wiss. Phot., 1905, 3, 58. 
CHANNoN—The Influence of Time on the Latent Image. Phot. J., 1917, 57, 72. 
Strauss—The Retrogression of the Latent Image. Kinotechnik, 1924, 6, 315. 


DEVELOPMENT AFTER FIXATION 


LEFFMANN—Development after Fixation. Brit. J. Phot., 1924, 71, 40. 

LUMIERE AND SEYEWETZ—Bull. Soc. franc. Phot., 1911, 2, 264, 373. 

LUMIERE AND SEYEWETZ—The Latent Image after Fixation. Compt. rend., 1924, 
179, 14. é 

LUMIERE AND SEYEWETZ—Development of the Latent Image after Fixing. 
Compt. rend., 1924, 178, 1765. 

LUMIERE AND SEYEWETZ—Causes of the Destruction of the Latent Image after - 
Fixation. Bull. Assn. Belg. Phot., 1924, 46, 74; S. I. P., 1924, 4, 130. 

Luppro-CraMER—Physical Development after Fixation. Phot. Ind., 1924, p. 780. 


604 PHOTOGRAPHY 


REVERSAL AND SOLARIZATION 


ARENS—Significance of Photographic Reversal. Zeit. Phys. Chem., 1925, 114, 
337- 

CiarK—Reversal by Sodium Arsenite, Hydrogen Peroxide and Light. Phot. 
J., 1924, 64, 363. . 

Eper—The Solarization of Photographic Plates. S. I. P., 1925, 5, 131; Brit. J. 
Phot., 1925, 72, 459. 

ENncLiscH—Studien uber die Solarisation bei Bromsilbergelatine. Arch. wiss. 
Phot., 1900, 2, 242. 

Lupro-CRAMER—Solarization. Phot. Ind., 1924, p. 1174; Zeits. Physik., 1924, 
29, 387. 

SCHEFFERS—Studies on Solarization. Zeits. Physik., 1923, 20, 100. 

SLATER-PriceE—On Solarization. Brit. J. Phot., 1925, 72, 506. 

TRIVELLI—Beitrag zur Kenntnis der Solarisationsphanomens und weiterer Eigen- 
schaften des latenten Bildes. Zeit. wiss. Phot., 1909, 6, 197, 237, 272. 


THEORIES OF THE LATENT IMAGE 


Asecc—Die Silberkeim Theorie oder subhaloid Theorie. Brit. J. Phot., 1899, 


46, 773; Phot. J., 1800, 39, April. 

ALLEN—The Formation of the Image on the Photographic Plate. Phot. J., 
1914, 54, 175. | 

- Bancrortr—The Latent Image. J. Phys. Chem., 1912; Brit. J. Phot., 1912, 59, 
881. 

Banxs—The Theory of the Latent Image. Brit. J. Phot., 1898, 45, 117. 

BoTtHAMLEY—The Latent Image. Phot. J., 1890, 39, Jan. 

Braun—Uber die Natur des latenten Bildes. Zeit. wiss. Phot., 1904, 2, 2090. 

Butt-—The Latent Image. Brit. J. Phot., 1906, 53, 160. 

Eper—Silber sub-bromide in latente Lichtbilde auf Bromosilber und die Silber- 
keimtheorie. Phot. Korr., 1899, 36, 276. . 

Eprr—Die Silberkeim Theorie und Verwachtes. Brit. J. Phot., 1899, 46, 788; 
Phot. Korr., 1899, 36, 1650. 

Emicuo—Zur Geschichte des latenten Photographischen Bildes. Zeit. wiss. 
Phot., 1907, 5, 183. 

HomotKka—The Latent Image and Developers. Brit. J. Phot., 1917, 64, 81. 

IpzErpA—Zur Theorie des latenten Bildes. Zeit. wiss. Phot., 1910, 8, 234. 

KincGpon—Considerations on the Nature of the Latent Image. Phot. J., 1906, 
46, 57; Brit. J. Phot., 1906, 53, 36. 

Luppo-CRAMER—Studien uber die Natur des latenten Lichtbildes. Phot. Korr., 
IQ0I, 39, 145, 218, 550, 643; Brit. J. Phot., 1901, 48, 520, 552, 569, 820; 
B. J. Almanac, 1903. 

Luppo-CRAMER—Neue Untersuchungen zur Theorie des photographischen Vor- 
gange. Phot. Korr., 1904, 42, 12, 118, 159, 254, 310, 374, 432, 478, 573. 

Luppo-CRAMER—Remarks on Some New Work on the Latent Image by Mr. F. 
F. Renwick. Phot. Korr., 1920, 57, 250, 285. 


Luppo-CrRAMER—History and Theory of the Latent Image. I. Zeit. wiss. Phot., 


1924, 23, 91; II. Ibid., 1925, 23, 122; III. Ibid., 1925, 23, 216. 


- —— 


; 
E 
i 


REFERENCES TO TECHNICAL JOURNALS 605 


LuTHEeR—Edersche Versuch und das latenten Bild. Brit. J. Phot., 1899, 46, 
664; Phot. Korr., 1899, 36, 584. 

LutTHER—On the Present State of our Knowledge of the Nature of the Latent 
Image. Brit. J. Phot., 1910, 57, 651. 

Mercator—The Nascent Silver and Sub-Haloid Theories. Brit. J. Phot., 1800, 
46, 628. 

ODENCRANTS—Was muss man von einer Theorie des latenten Bildes fordern. 
Zeit. wiss. Phot., 1919, 16, 261. 

Rawiinc—The Mystery of the Latent Image. Phot. J., 1923, 63, 482. 

RENWIcK—Photographic Images—Visible and Invisible. Brit. J. Phot., 1920, 


67, 447, 463. 

ReENwicK—The Gelatine Emulsion and the Latent Image. Brit. J. Phot., 1923, 
70, 382. 

ScHAuM—Zur Theorie des Photographischen Processe. Arch. wiss. P., 1900, 
2, 9. 


ScHUMANN—Theory of the Latent Image. Phot. J., 1890, 39, 313. 
SEYEWETZ—The Latent Image. Chemie et Industrie, 1925, 13, 355. 
STEIGMANN—Theory of Photographic Light Sensitivity. Chem.-Ztg., 1924, 48, 
234. 
THORNE-BAKER—Cause of Sensitivity of Silver Bromide Emulsions. Phot. J., 
| 1924, 64, 369. 

TRIVELLI—Beitrag zur Kenntnis der Silberhaloide. Zeit. wiss. Phot., 1909, 6, 
358, 438. 

' TRIVELLI—Beitrag zur Photochemie der Silber (sub) haloide. Zeit. wiss. Phot., 
IQII, 8, 113. 

TRIVELLI, SHEPPARD AND LovELAND—The Formation of the Latent Image. J. 
Frank. Inst., 1925. 

Toy—The Mechanism of the Latent Image. The Physical Chemistry of the 
Photographic Process. (Published as the Transactions of the Faraday 
Society.) 

We1sz—Researches on the Latent Image by Means of Plates Free from Colloid. 
Brit. J. Phot., 1907, 54, 960. 


Chapter IX. Sensitometry 


(For list of general reference works see page 253) 


GENERAL PAPERS ON SENSITOMETRY 


Bioco—Sensitometry. The Physical Chemistry of the Photographic Process. 
(Published as the Transactions of the Faraday Society.) 

BrowNn—The H. and D. Doctrine. Brit. J. Phot., 1921, 68, 335, 354, 372, 386, 
“401, 415. 

Eprer—System der Sensitometrie Photographischen Platten. Phot. Korr., 1900, 
37, 241, 304, 364, 441, 495, 567, 625; 1902, 39, 386, 449, 504. 

MerrEs AND SHEPPARD—Instruments for Sensitometric Investigation. Phot. J., 
1904, 44, 200. (With excellent bibliography.) 

MEES AND SHEPPARD—The Sensitometry of Photographic Plates. Phot. J., 
1904, 44, 282. (With excellent bibliography.) 
40 


606 PHOTOGRAPHY 


ODENCRANTS—Sensitometrische Apparate und deren Fahlerquellen. Zeit. wiss. 
Phot., 1919, 16, 69 (Bibliography). 


STANDARD LiGHT SOURCES 


BoTHAMLEY—The Amyl-Acetate Lamp. Phot. J., 1894, 34, 231. Eder’s Jahr- 
buch, 1890, 54. 

Dispin—Light Standards. Phot. J., 1894, 6, 712. 

EpEr—Employment of Magnesium as a Secondary Source. Brit. J. Phot., 1925, 
ype BW SNS To. LR Se 

EncLtisH—Eine Amyl-Acetate lampe fur Sensitometrische Zwecke. Phot. Mitt., 
IQOI, 28, 157. 

Fasry—Luminous Standards for Sensitometry. S. I. P., 1925, 5, 121. 

Frery—An Acetylene Standard. Phot. J., 1905, 45, 132. 

Jonges—Light Standards for Sensitometry. S. I. P., 1925, 5, 123. 

Jouaust AND BatLaup—Color Temperature of the Acetylene Flame. S. I. P., 


1925, 5, 124. 
Jouaust—The Incandescent Lamp as a Sensitometric Standard. S. I. P., 1925, 
5, Taz 


LuTHER—Constant Light Source. Phot. J., 1925, 65, 60. 

Merrs—Screened Acetylene Light. Brit. J. Phot., 1906, 53, 890. 

MEES AND SHEPPARD—Investigations on Standard Light Sources. Phot. J., 
1910, 50, 287; Brit. J: Phot., 1910, 57, 627. 

NauMANN—Artificial White Light for Photographic Purposes. Phot. J., 1925, 
65, 348. 

NAuMANN—Colored Filters for Sensitometric Light Standards. Zeit. wiss. 
Phot., 1925, 23, 303. 

WatsH—Standards of Light for Photographic Sensitometry. Phot. J., 1925, 
65, 52. 


Exposing APPARATUS 


BRIEFER—Improvements in the Disk Densitometer of H. and D. Trans. Motion 
Picture Engineers, 1925, No. 21, p. 85. 


CALLIER—The Construction of Photometric Instruments. Phot. J., 1913, 53, 


242; Brit. J. Phot., 1913, 60, 951, 972. 
Davipson AND BaLMAIN—A New Form of Exposing Apparatan Phot... J; 


1925, 65, 60. 
Eprr—The Eder-Hecht Wedge in Sensitometry and Photometry. Phot. Korr., 
1920, 56, I, 41. 
Fasry—The Measurement of Density by Photographic Methods. S. I. P., 
1925, 5, 128. . 


Harpy—A Non-intermittent Sensitometer. Jl. Opt. Soc. Amer., 1925, 10, 149. 

Hicson—A Simple Non-intermittent Sensitometer. Phot. J., 1920, 60, 235. 

Hitcuins—A Non-intermittent Sensitometer. Bull. Soc. franc. Phot., 1921, p. 
74. 

Jones—A Simple and Inclusive Method of Testing Pilates. (Chapman Jones 
plate speed tester.) Phot. J., 1901, 41, 246; Eder’s Jahrbuch, 1901, 15, 
491. 


REFERENCES TO TECHNICAL JOURNALS 607 


Jones—A Non-intermittent Sensitometer. Phot. J., 1920, 60, 60. 

RAWLING—Exposure Mechanism. Phot. J., 1925, 65, 64. 

RENwicK—Sources of Error and Differences in Dry Plate Sensitometers. Brit. 
J. Phot., 1910, 57, 626. 

RENwick—Note on Exposure Mechanisms. Phot. J., 1925, 65, 74. 


EFFECT OF INTERMITTENT EXPOSURE AND THE RELATION OF TIME TO INTENSITY 


AsnrEY—The Effect of Intermittent Exposure and the Relation between Time 
and Intensity. Treatise on Photography, 1903, p. 391. 

Bitocuo—Plate Speeds, Failure of the Reciprocity Law. Phot. J., 1917, 57, 51. 

Husit—Determination of Schwarzschild’s Index and its Significance. Phot. 
Korr., 1919, p. 363. 

Jones AND Huse—On the Relation between Time and Intensity in Photographic 
Exposure. J. Opt. Soc. Amer., 1923, 7, III5. 

MA.LLET—Photographic Plates and the Law of Schwarzschild. S. I. P., 1923, 


3, 1. 

RENwicK—Some Deductions from Schwarzschild’s Rule. Phot. J., 1916, 56, 
II. 

Ropertson—Determination of the Schwarzschild Constant. Jl. Opt. Soc. Amer., 
1923, 7, 900. 


ScHWARZzSCHILD—Uber die Wirkung intermittentender Belichtung auf Bromsil- 
bergelatine. Phot. Korr., 1899, 36, 109, 171. 

StrAuss—The Schwarzschild Exponent. Kinotechnik, 1924, 6, 125. 

WALLACE AND LeEmon—The Reciprocity Law. Brit. J. Phot., 1909, 56, 378. 

WERNER—Das photographische Reziprozitatsgesetz fur Bromsilbergelatine bei 
Erregung mit Licht verschiedener Wellenlauge. Zeit. wiss. Phot., 1907, 
5, 382; 1908, 6, 25. 


DENSITOMETERS AND Density MEASUREMENT 


BaKker—A Photo-Electric Photometer and Densitometer. J. Scient. Inst., 1924, 
I, 345. 

BuLt AND CarTWRIGHT—The Measurement of Photographic Density. Phot. J., 
1924, 64, 180. 

Butt AND CartwricHt—An Evaluation of the Light Scattered by Photographic 
Densities. Phot. J., 1925, 65, 177. 

CALLIER—Absorption and Scatter of Light by Photographic Negatives Meas- 
ured by Means of Marten’s Polarization Photometer. Phot. J., 1900, 49, 
200; Zeit. wiss. Phot., 1909, 7, 257. 

CAPSTAFF AND GREEN—A Motion Picture Densitometer. Phot. J., 1924, 64, 97. 

CarNEGIE—Modification of the H. and D. Photometer. Brit. J. Phot., 19009, 

56, 197. 

CLavieR—Influence of Non-uniformity on Photographic Plates on Photometric 
Measurements. S. I. P., 1924, 4, 9. 

Cousin—A New Photographic Photometer. Brit. J. Phot., 1907, 54, 24. 

CLERc—The Measurement of Density and the Expression of the Results. S. I. 
Eo 1oes, 6 128... 


df 


ee Uo ae , 


608 | PHOTOGRAPHY 


Dozsson—A Flicker Type of Photo-electric Photometer. Proc. Roy. Soc., 1923, 
A104, 248. 

EccErRT AND ARCHENHOLD—The Optical Scattering Power of Photographically 
Developed Silver Layers. Z. physik. Chem., 1924, 110, 407. 

Frercuson—A New Density Meter. Phot. J., 1911, 51, 405; Brit. J. Phot., 1912, 
59, 24; Eder’s Jahrbuch, 1912, 26, 469. 

Frercuson—A Bar Photometer for Measuring Densities by Non-parallel Light. 
Phot. J., 1912, 52, 283; Brit. J. Phot., 1912, 59, 772. 

Frercuson—The F. R. B. Photometer. Phot. J., 1918, 58, 155; Brit. J. Phot., 
1918, 65, 128, 213, 224. 

Fercuson—The Ferguson Density Meter No. V. “Phot. J., 1924, 64, 30; Brit. 
J. Phot., 1924, 71, 10. 

GoLpBERG—The Densograph. Brit. J. Phot., 1910, 57, 649; Eder’s Jahrbuch, 
IQIO, 24, 226. 

HartMAn—A New Photographic Photometer. Brit. J. Phot., 1900, 47, 67. 

Harrison—Precision Photometers for Photographic Photometry. Jl. Opt. Soc. 
Amer., 1925, 10, 157. 

HorFMANN—Ein neues Photometer zur Sensitometrie. Phot. Korr., 1901, 38, 


QI, 651. 
Hyort, Lowy anp Biackwoop—Optical Densitometer. JI. Opt. Soc. Amer., 
1924, 9, 43. 


Jones—A Densitometer for the Measurement of High Photographic Densities. 
J. Opt. Soc. Amer., 1923, 8, 231. 

Jones—An Opacity Balance. Phot. J., 1898-0, 38, 90. 

Lux—A Densitometer. Phot. Korr., 1920, p. 13. 

Martens—Modified Konig Photometer. (Exact title not available.) Phot. 
Korr., 1901, 39, 528. 

PrrrRINE—Photo-electric Cell Photometer for the Measurement of Photographic 
Densities. Jl. Opt. Soc. Amer., 1924, 8, 381. 

Prunp—The Pfund Photometer. Brit. J. Phot., 1907, 54, 660. 

PatuEe—Influence of the Diffusion of Light in Photometric Miaharanients 
Brit. J. Phot. i025, 723°S.. 1. P., 1085, soa 

ReNwick—A New Form of Density Measuring Apparatus. Phot. J., 1910, 50, 
177, 

ReNwick—The Measurement of Densities. Phot. J., 1912, 52, 250, 260; Brit. 
J. Phot., 1912, 59, 304; Eder’s Jahrbuch, 1914, 28, 384. 

RenwickK—The Effect of Inter-reflection on Density Values. Phot. J., 1013, 
53, 204; Brit. J., 1913, 60, 611. 

RENwickK—An Improved Form of the Ferguson Bench Photometer. Phot. J., 
1914, 54, 167. 

Renwick—An Instrument for the Measurement of Gamma. Phot. J., 1914, 54, 
163. 

RENwick—How Should the Densities of a Photographic Deposit be Measured. 
Brit. J. Phot., 1924, 71, 65. 

SANGER-SHEPPARD—Density Meter. Brit. J. Phot., 1911, 58, 926; British Patent 
23,429 of IgII. 

STENGER AND Kuyawa—The Measurement of Photographic Density. Zeit. 
wiss. Phot., 1924, 23, 80; Phot. Ind., 1924, p. 953. 


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REFERENCES TO TECHNICAL JOURNALS 609 


Toy AnD Rawiinc—Photo-electric Densitometer. Phot. J., 1924, 64, 189; Jl. 
Scient. Instr., 1924, 1, 362. 
Toy—The Standardization of Photographic Density Measurements. Phot. J., 


1925, 65, 164. 
Weser—Causes of Error in Density Measurements. Zeit. wiss. Phot., 1924, 
23, 175. 


Wiisry—Correction for Fog in Photographic Densities. Phot. J., 1925, 65, 454. 


DEVELOPERS AND DEVELOPMENT OF SENSITOMETRIC TESTS 


Braver—New Methods of Control for Thermostats. J. Ind. Eng. Chem., 1923, 
15, 359. 

CLarK—Standard Development. Phot. J., 1925, 65, 76. 

HARRISON AND Dozgson—Note on the Uniform Development of Photographic 

Pilates, Phot. J., 1925, 65, 80. 

MerEES AND SHEPPARD—Instruments for Sensitometric Investigation. Phot. J., 
1904, 44, 210. 

SHEPPARD AND Exrriotr—lInfluence of Stirring on the Rate and Course of De- 
velopment. J. Frank. Inst., 1924, 198, 333. 


WEDGE SCREENS AND THEIR USE IN SENSITOMETRY 


Biocx—Selective Absorption of Neutral Wedges. Phot. J., 1917, 57, 51. 

Eprr—Application of the Neutral Wedge Sensitometer. Phot. Korr., 1922, p. 
17 (festnummer). 

GoitpBerc—Gelatine Wedges. Brit. J. Phot., 1910, 57, 642, 648. 5 

GoLtpperc—Uber die Automatische Herstellung der Characterischen Kurve. 
Zeit. wiss. Phot., 1911, 9, 323; Brit. J. Phot., 1910, 57, 642, 664. 

Hicson—Wedge Method of Photometry. Phot. J., 1921, 61, 93. 

LANnGcEeR—Neutral Wedge Sensitometry according to the Normal Constant Sys- 
tem. Phot. Rund., 1924, 61, 59. 

LEHMANN—Neutral Wedges in Sensitometry. Zeit. wiss. Phot., 1922, p. 214 
(festnummer). 

Lopet—Automatic Registration of the Characteristic Curve. Bull. Soc. franc. 
Phot., 1924, 11, 209. 


THE INTERPRETATION OF RESULTS 


BaAKeER—Interpretation of the Characteristic Curve. Phot. J., 1925, 65, 181. 

BiocH—Interpretation of Results. Phot. J., 1925, 65, 186. 

Eprr—For all papers see Beitrage zur Photochemie. 

HerypDEcKER—Rapid Solution of Some Common Problems in Sensitometry. S. 

eet 1OR6, 5, 21. 

LutTHER—Interpretation of the Characteristic Curve. Phot. J., 1925, 65, 185. 

Meres—The Interpretation of Sensitometric Tests. Brit. J. Phot., 1906, 53, 104, 
126, 143, 179, 617, 636, 797, 857. 

Merrs—Report on the Present Condition of Sensitometry. Brit. J. Phot., 1909, 
56, 685. 


610 : PHOTOGRAPHY 


Mrres—The Photographic Reproduction of Tone. Phot. J., 1924, 64, 310. 

RayLeicgH—The General Problem of Photographic Reproduction. Phil. Mag., 
IQII, p. 734; Brit. J. Phot., 1911, 58, 904. 

RenwickK—The Under Exposure Period in Theory and Practice. Phot. J., 1913, 
52. 127% . 

Renwick—Tone Reproduction and its Limitations. Phot. J., 1916, 56, 222; 
Brit. J. Phot., 1916, 63, 675. 

ReENwicK—The Under Exposure Period. Brit. J. Phot., 1912, 59, 280, 312; 
Eder’s Jahrbuch, 1912, 26, 106. 

THORNE-BAKER—Interpretation of the Characteristic Curve. Phot. J., 1925, 
65, 181. 

Wartxkins—New Methods of Speed and Gamma Testing. Phot. J., 1912, 52, 
207; Brit. J. Phot., 1912, 59, 316. 

Watit—Elementary Sensitometry. Amer. Phot., 1923, pp. 298, 356, 416. 


j 


Chapter XI. The Theory of Development 
(For list of general reference works see page 288) 


On THE THEORY OF DEVELOPMENT 


Asecc—Eine Theorie der photographischen Entwicklung. Arch. Wiss. P., 
1899, I, 109. 

AsecGc—Theorie des Eisenentwicklers nach Luther. Arch. Wiss. P., 1900, 2, 
76. 

Asecc—Zur Frage nach der Wirkung der Bromide auf die Entwickler. Eder’s 
Jahrb., 1904, 18, 65. 

ANDRESEN—Zur Theorie der Entwicklung des latenten Lichtbildes. Phot. Korr., 
1898, 35, 445. 

ARMSTRONG—The Chemical Changes Attending Photographic Operations. I. 
The Theory of Development in Relation to the Essentially Electrolytic 
Character of the Phenomena and: the Nature of the Photographic Image. 
Brit. J. Phot., 1892, 39, 276. 

Bancrort—The Effect of Bromide. Brit. J. Phot., 1912, 59, 878. 

Banxs—The Theory of Development. Brit. J. Phot., 1896, 43, 677. 

BoTHAMLEY—Remarks on Some Recent Papers on the Latent Image and its — 
Development. Phot. J., 1890, 39, 123. 

Brepic—Die electromotorische Scala der photographischen Entwicklers. Eder’s 
Jahrb., 1895, 9, 19. 

DrsALME—The Chemical Theory of Development. Brit. J. Phot., 1910, 57, 653. 

FRIEDLAENDER—Zur Theorie der Entwicklung. Phot. Korr., 1902, 39, 252. 

Hurrer AND DrirFieLD—The Action of Potassium Bromide. Phot. J., 1898, 
38, 360. 

KELLER—The Theory of Photographic Development. Koll. Zeits., 1923, 32, 304. 

KroHN—The Mechanism of Development of the Image in a Dry Plate Nega- 
tive. Phot. J., 1918, 58, 179; Brit. J. Phot., 1918, 65, 412. 

LUMIERE AND SEYEWETZ—Contribution a,L’Etude du Role des alcalis dans les 
Revelateurs organiques. Bull. Soc. franc. Phot., 1906, 16, 32. 


a ae ee ee eS = se CULT le 


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REPERENCES TO TECHNICAL JOURNALS 611 


Luppo-CRAMER—Die verzogernde Wirkung der Bromide in den photograph- 
ischen Entwicklern als kolloidchemischer Vorgang. Koll. Zeits., 1900, 
4, 92. 

Luppo-CRAMER—Uber den Einfluss der Bromide im Entwickler auf die topo- 
graphische Verteilung des Silbers im Negativ. Phot. Korr., 1912, 49, 383. 

Luppo-CrRAMER—Ueber die beranderung der konform der bromsilbers bei der 
Reduktion und die Nahronertheorie der Entwicklung. Phot. Korr., 1911, 


48, 547. 
Lupro-CrRAMER—Zur theorie der chemischen Entwicklung. Phot. Korr., 1908, 
35, 206. | 
Luppo-Cramer—Acceleration of Development by Soluble Iodides. Koll. Zeits., 
1922, 30, 186. 


MATTHEWS AND BARMEIER—The Electro-potentials of Certain Photographic De- 
velopers and a Possible Explanation of Photographic Development. 
Brit. J. Phot., 1912, 59, 879. 

Merrs—Time Development. Phot. J., 1910, 50, 403; Brit. J. Phot., 1910, 57, 
919; Eder’s Jahrb., 1910, 25, 161. 

Pinnow—Behaviour and Function of Sulphite in Developing Solutions. P. 
Rund., 1923, 60, 27. 

PrecHT—Beitrage zur Theorie der Photographischen Entwicklung. Arch. 
Wiss. P., 1900, 2, 155; Brit. J. Phot., 1900, 47, 650. 

Rees—Theory of Development. Phot. J., 1906, 46, 302; Bull. Soc. franc. Phot., 
1904, 20, 324. 

ReNnwick—The Physical Process of Development. Brit. J. Phot., 1911, 58, 75. 

ScHAaumM—Zur theorie des photographischen prozesses. I. Das latenten Bild; 
II. Der Entwicklungsvorgang. Arch. Wiss. P., 1900, 2, 9. 

ScHILOW AND TIMTSCHENKO—Physikalisch-chemische Studien an photograph- 
ischen Entwicklung. JII. Hydrochinon als Induktor. Zeits. Elektro- 
chem., 1913, 19, 8106. 

SHEPPARD—Theory of Alkaline Development, with Notes on the Affinities of 
Certain Reducing Agents. J. Chem. Soc., 1906, 89, 530. 

SHEPPARD—Reversibility of Photographic Development and the Retarding Ac- 
tion of Soluble Bromides. J. Chem. Soc. (London), 1905, 87, 1311; 
Zeit. wiss. Phot., 1905, 3, 443. 

SHEPPARD—The Electro-chemistry of Development. Trans. Electrochem. Soc., 


I92I, , Pp. 429. . 
SHEPPARD—On the Silver Germ Theory of Development. Phot. Korr., 1922, 
59, 76. | 


SHEPPARD AND Ettiotr—On the Theory of Development. Trans. Faraday Soc., 
1923, 19, 355. 

SHEPPARD AND Mers—On the Chemical Dynamics of Development. Phot. 

' J., 1905, 45, 281; Zeit. wiss. Phot., 1904, 3, 97. 

SHEPPARD AND Meres—Some Points in Modern Chemical Theory and their 
Bearing on Development. Phot. J., 1915, 45, 241. 

SHEPPARD AND Mryer—On Chemical Induction in Photographic Development. 
J. Amer. Chem. Soc., 1920, 42, 680. 

VotMER—The Theory of Development of the Latent Photographic Image. 
Zeit. wiss. Phot., 1921, 20, 189; Phot. Korr., 1921, p. 226. 


612 : PHOTOGRAPHY 


On THE PHYSICAL CHEMISTRY OF PHOTOGRAPHIC DEVELOPMENT 


Birocu—Plate Speeds. Phot. J., 1917, 57, 51. 

DRIFFIELD—Control of the Development Factor. Phot. J., 1903, 43, 17. 

HurTerR AND DrirrieLD—Photochemical Investigations. J. Soc. Chem. Ind., 
1890, 9, 455. 

HuRTER AND DrIFFIELD—Exposure and Development. Phot. J., 1805, 35, 372. 

HurtTER AND DrirFieLD—The Latent Image and its Development. Phot. J., 
1898. 

FERGUSON AND Howarp—Control of the Developing Factor at Various Tem- 
peratures. Phot. J., 1905, 45, 118; Brit. J. Phot., 1905, 52, 249; Eder’s 
Jahrb., 1905, 21, 408. 

Frercuson—A New Method of Calculating the Time of Development at Various 
Temperatures. Phot. J., 1906, 46, 182; Brit. J. Phot., 1906, 53, 206; 
Eder’s Jahrb., 1905, 21, 474. : 

FEercuson—Investigations on the T.C. of a Pyro Soda Developer. Phot. J., 
IQ10, 50, 412; Eder’s Jahrb., 1910, 25, 506; Bull. Soc. franc. Phot., rozo, 
27, 175. 

KroHn—The Mechanism of Development of the Image in a Dry Plate Nega- 
tive. Phot. J., 1918, 58, 179; Brit. J. Phot., 1918, 65, 412. 

LuTHER—The Physical Chemistry of Negative Processes. Phot. J., 1912, 52, 
201. 

Mers—Interpretation of Sensitometric Tests. Brit. J. Phot., 1906, 53, 104, 126, 
143, 179, 617, 636, 797, 857. 

Merres—Time Development. Phot. J., 1910, 50, 403; Brit. J. Phot., 1910, 57, 
919; Eder’s Jahrb., 1910, 25, 161. 

Merrs—Physical Chemistry of Photographic Development. Brit. J. Phot., 1913, 
60, 935. 

MeEES AND SHEPPARD—On the Development Factor. Phot. J., 1903, 43, 48. 

MEES AND SHEPPARD—On the Highest Development Factor obtainable on any 
Plate. Phot. J., 1903, 43, 199. 

MEES AND SHEPPARD—On the Sensitometry of Photographic Plates. Phot. J., 
1903, 43, 199. 

MEES AND SHEPPARD—Some Points in Modern Chemical Theory and their 
Bearing on Development. Phot. J., 1905, 45, 241. 

Nietz—Theory of Development. Phot. J., 1920, 60, 280. 

Nietz—Theory of Development. Monograph No. 2 Eastman Research Lab- 
oratory, 1922. 

Pinnow—The Function of Sulphite in Alkaline Developers. P. Rund., 1923, 
60, 27. 

Prper—Application of Physico-chemical Theories in Plate Testing and Experi- 
mental Work with Developers. Brit. J. Phot., 1913, 60, 110. 

REeNwick—The Physical Process of Development. Brit. J. Phot., ror1, 58, 75. 

Renwick—The Calculation of Gamma Infinity. Phot. J., 1911, 51, 213; Bull. 
Soc. franc. Phot., 1911, 352. 

Renwick—An Improved Method of Computing the Velocity Constant and 
Gamma Infinity. Phot. J., 1923, 63, 331. 

SHEPPARD—The Chemical Dynamics of Photographic Development. Proc 
Royal Society, 1904, 74, 457. 


REFERENCES TO TECHNICAL JOURNALS 613 


Toy anp Hicson—Factors Determining Gamma Infinity. Phot. J., 1923, 63, 
68; S. T. I. P., 1923, 3, 131. 

Watxins—On the Variation of the Temperature Coefficient with Different 
f iatege 2) NOt. )4 1010, 50, 411; Brit. J. Phot., 1911, 58, 3. 

Watxins—New Methods of Speed and Gamma Testing. Phot. J., 1912, 52, 
200. 


Chapter XII. The Organic Developing Agents 
ON THE CONSTITUTION OF ORGANIC DEVELOPING AGENTS 


ANpDRESEN—Constitution der Entwickler. P. Mitt. 1891, 27, 124, 286, 206. 

ANDRESEN—Constitution organischer Entwickler. Eder’s Jahrb., 1893, 7, 418. 

ANDRESEN—Organische Entwicklersubstanzen. Eder’s Jahrb., 1903, 7, 486. 

ANDRESEN—Zur Charakterisirung der Entwicklersubstanzen. Phot. Korr., 1890, 
36, 635. 

ANDRESEN—Zur Chemie der organischen Entwickler. Phot. Korr., 1900, 37, 185. 

Homo.tka—Beitrage zur Theorie der organischen Entwickler. Phot. Korr., 
1914, 51, 256, 471. 

Homo,txa—The Latent Image and Development. Brit. J. Phot., 1917, 64, 81. 

LrerppeER—Photographic Developers. Brit. J. Phot., 1900, 47, 826. 

LirsEGANG—Die Constitution der organischen Entwickler. Photochemischen 
Studien, vol. II, 1894. 

Lopet—Der Ersatz der Alkalien durch Ketone und Aldehyde in den photo- 
graphischen Entwicklern. Eder’s Jahrb., 1904, 18, 103. 

LUMIERE AND SEYEWETZ—Die Bildung von Salzen mit Entwicklerfahigkeit aus 
Aminen und Phenolen. Eder’s Jahrb., 1899, 13, 306. 

LUMIERE AND SEYEWETZ—On the Developing Power of Hydrochinon Substitu- 
tion Compounds. Brit. J. Phot., 1914, 61, 341. | 

LUMIERE AND SEYEWETzZ—Sur les Substitutions Alkylees dans les Groups de la 
Function Developpatrice. Bull. Soc. franc. Phot., 1898, 14, 158. 

LUMIERE AND SEYEWwETz—Ueber die Additionsprodukte, welche die Gruppen 
mit entwickelnden Eigenschaften mit den Aminen und Phenolen bilden. 
Arch. wiss. Phot., 1899, I, 64. 
LuMizRE AND SEYEWETZ—Influence du Groupe Cetonique sur le Pouvoir De- 
veloppateur des Polyphenols. Bull. Soc. franc. Phot., 1897, p. 415. 
LUMIERE AND SEYEWETz—Sur la Constitution des Substances Reductrices, sus- 
ceptibles de developperd L’Image Latente sans entre additionees D’ Alkali. 
Bull. Soc. franc. Phot., 1903, ; Eder’s Jahrb., 1904, 18, 99. 

LUMIERE AND SEYEWETZ—Untersuchungen uber die chemische Konstitution der 
Entwickler-Substanzen. Eder’s Jahrb., 1898, 12, 100. 

LUMIERE AND SEYEWETZ—Sur la Fonction Developpatrice. Bull. Soc. franc. 
Phot., 1806, p. 268. 

SrYEWETz—The State of our Knowledge of Organic Developing Agents. Bull. 
Soc. franc. Phot., 1920, p. 129; Brit. J. Phot., 1920, 66, 186. 


614 . PHOTOGRAPHY 


On OrcGANIC DEVELOPING AGENTS 


“ Acra ’”’—Photographic Developers. Paraphenylene Diamine with Amido or — 
Hydroxyl Groups. British Patent 11,872/1803. * q 
“ Acra ”—Photographic Developers. Para-oxy-phenyl-glycinamide. British _ 
Patent 9537/1905. q 
“ Acra ”’—Photographic Developers. Oxy-phenyl-alkyl-glycin. British Patent — 
18,095/1913. 7 
ANDRESEN—Photographic Developers of the Naphthalene Series. British Pat- — 
ent 5207/1889. a 
ANpbRESEN—Photographic Developers. Amido Derivatives of Naphthol. Brit- — 
ish Patent 25002/1893. a 
ANDRESEN—Bromhydrochinone als Entwickler. Phot. Korr., 1890, 36, 306. 
ANDRESEN—Die Isomeren des Amidols. Phot. Korr., 1894, 31, 505. | 
ANDRESEN—Verwendung von Derivaten des p-Phenylendiamins, sowie des p- — 
Toluylendiamins als Entwickler in der Photographie. Eder’s Jahrb., 1895, — 
9, 50. 
ANDRESEN—Weitere Beitrage zur Kenntniss des Diamidooxydiphenyls als — 
Entwickler. Phot. Korr., 1899, 36, 208. 
BUCHERER—Photographic Developers of Paratoluolsulfonylaminophenol. D. R. 
P. 369,391/1921. 
BuneLt—Preservation of Diamidophenol Developers in Solution. Il. Prog. Fot., 
192I, p. 204. 
CROWTHER—Preservatives of Amidol. Brit. J. Phot., 1920, 67, 642. 
DrEsALME—On a New Developer, Sulphinol. Brit. J. Phot., 1912, 59, 425. 4 
DieTerLE—Photographic Developer of Sulpho-acid Aminophenol. United — 
States Patent 1,432,542/1922. 7 
Druce—Stabilizing Solutions of Amidol. Brit. J. Phot., 1922, 69, 81. 
ErMEN—Rodinal Type Developers. Brit. J. Phot., 1920, 67, 611. 3 
ErMEN—Preparation of Monomethyl Paramidophenol Sulphate (Metol). Phot. — 
J., 1923, 63, 223. * 
FaucHEy—Conservation of Amidol Developers. Bull. Soc. franc. Phot., 1923, — 
10, 90. 
FiscHer—Developers Yielding Colored Images. British Patent 2562/1913. 
Grear—A New Developer (Ds50). Brit. J. Phot., 1921, 68, 307. 
Havurr—Amidophenol Developers. British Patent 15,434/1891. ‘_ 
Haurr—Photographic Developer of Ortho-para-diamidophenol Ortho-p-diam- — 
ido-ortho-cresol-p-diamidometa-cresol. British Patent 14,542/1892. 4 
Haurr—Photographic Developers of Sulphonic or Carboxylic Acids of Ortho, 
or Para, or Ortho-para, Amidophenol (Neol). British Patent 154,198 of — 
1920. q 
Homo_tkA—Hydrocoerulignon as a Developer. Phot. Korr. (Festnummer), 
1922; Brit. J. Phot., 1922, 69, 397. 
Kinc—Photographic Developers of 2: 4 Diamidophenol and Stannous Chloride. 
British Patent 196,672/1922. 
Loset—Preservatives of Amidol in Solution. Brit. J. Phot., 1921, 68, 7o1. 
LUMIERE AND SEYEWETZ—Sur les Properties Developpatrices des Hydroxyl- 
amines Aromatiques. Bull. Soc. franc. Phot., 1894, , 487. 


REFERENCES TO TECHNICAL JOURNALS 615 


LUMIERE AND SEYEWETZ—Sur les Proprietes Revelatrices D’une Nouvelle Com- 
binaison D’hydrochinone et de Paraphenylenediamine. Bull. Soc. franc. 
Phot., 1890, , 135- 

LUMIERE AND SEYEWETz—Sur la Preparation et les Proprietes Revelatrices de 
la Metoquinone-combinaison de metol et D’hydrochinone. Bull. Soc. 
francPhot,, 1903, , 231. 

LUMIERE AND SEYEWETZ—Sur la Preparation et les Proprietes Revelatrices die 
Chloranol (Chlorhydroquinonemethylparamidophenol). Bull. Soc. franc. 
Phot., 1913, p. 223. 

Luppro-CrAMER—Development with Amidol and Related Substances. Phot. 
Korr., 1921, 68, 121 (Festnummer). 

MeELpoLa—Eikonogen. J. Soc. Chem. Ind., 1889, , 958. 

Perk1N—Pyrocatechin. J. Chem. Soc., 1890, 57, 587. 

SCHERING—Benzyl Para-amidophenol Compounds as Developers. British Pat- 
ent 20,050/1907. 

STEWART—Photographic Developers of Aminophenol Derivatives. Canadian 
Patent 237,842/1920. 

“THeRmiIT ”"—Glycollic Acid as a Preservative of Amidol. Brit. J. Phot., 1921, 
68, 125. 

VALENTA—Das Sulfinol als Entwickler fur Bromsilbergelatine Trockenplatten. 
Phot. Korr., 1915, 52, 206. 

VALENTA—4: Oxyphenylmethylglycin als Entwicklersubstanz. Phot. Korr., 
1915, 52, QO. 

VALENTA—Ueber die Verwendbarkeit von Diamidophenolnatrium zur Entwick- 
lung von Bromsilbergelatine Trockenplatten. Eder’s Jahrb., 1905, 169, 
122. 


MISCELLANEOUS 


CLARK—Chemical Tests for Developing Substances. Brit. J. Phot., 1918, 65, 
499. 

CRABTREE—Photographic Methods of Testing Developers. American Annual 
of Photography, 1922, p. 184. 

DuUNDON AND CRABTREE—Fogging Properties of Developers. Brit. J. Phot., 
1924, 71, 701, 719. 

ErMEN—Tests for Developing Agents. Brit. J. Phot., 1917, 64, 390. 

Kamu—The Solubility of Developing Agents. Phot. Korr., 1921 (Festnummer). 

LUMIERE AND SEYEWETZ—Influence de la Nature des Revelateurs sur la Gros- 
seur de Grain de L’Argent Reduit. Bull. Soc. franc. Phot., 1904, , 294. 

LUMIERE AND SEYEWETz—Sur Procede de Developpement Photographique con- 
duisant a L’Obtention D’Images a Grain Fin. Bull. Soc. franc. Phot., 
1904, » 422. 

MEEs AND Piper—The Fogging Power of Developers. Phot. J., 1911, 51, 226; 
1912, 52, 221. Brit. J. Phot., 1911, 58, 312, 491, 515; I912, 59, 337, 342, 
428, 441, 465. Bull. Soc. franc. Phot., 1912, p. 44. 


616 PHOTOGRAPHY a 


Chapter XIII. The Technique of Development ae : 
(For list of general reference works see page 337) 


ON THE TECHNIQUE OF DEVELOPMENT 


ALvEs—Time-Development: Its Excellences and Abuses. Brit. J. Phot., 1910, © 
57, 378. | - 
Amor—Desensitizers and Chemical Fog. Brit. J. Phot., 1925, 72, 183. 4 
BayLEy—Time Development. Brit. J. Phot., 1905, 52, 140, 168. | 4 ! 
BoTHAMLEY—Fundamental Points on Development. Brit. J. Phot., 1809, 46, 
453; Arch. wiss. Phot., 1900, 2, 24; Bull. Soc. franc. Phot., 1900, 15, 520. 
CLEVELAND—Desensitizing with Phenosafranine. Amer. Phot., 1922, 16, 756. | z 
CrowTHER—Pinakryptol and Developers. Brit. J. Phot., 1922, 69, 351. 
Dawson—Time Development with the B. J. Pyro-Soda Formula. Brit. j.3 
Phot., 1915, 62, 445. 
ErmMen—The Effect of Safranine on Development. Brit. J. Phot., 1921, 68, 7 
445. . 
EIS HES os of Calculating the Time of Development at Various Tem- 4 
peratures. Phot. J., 1906, 46, 182; 1910, 50, 412. 4 
GLoverR—A Comparison of Develonment Methods. Brit. J. Phot., 1921, 63, i 
183, 195. a 
HomoLtxa—New Desensitizers. Phot. Ind., 1925, p. 347. a 
Husit—Contribution to Our Knowledge of Desensitizing. Phot. Hund 1925, 62, 
71. 4 
Hupsit—Methylene Blue as a Desensitizer. Phot. Ind., 1925, p. 14. 
Krncpon—Causes of Variation in the Watkins Factor for Different Developers. 
Phot. J., 1918, 58, 270. a 
von Kienck—Thermo-Entwicklung. Phot. Mitt., 1902, 39, 232. ; 
Krart—Time Development. Amer, Phot., 1922, 16, 1; Brit. J. Phot., 1922, 69, 
123. a 
Locxett—The Personal Element in Factorial Development. Brit. J. Phot., 1906, 
53, 464, 502. — 
LUMIERE AND SEYEWETZ—Les Succedanes des ‘Atraila dans les Develonps ea ; 
Alcalins. Bull. Soc. franc. Phot., 1895,  , 32. 4 
LUMIERE AND SEYEwETz—Sur L’Emploi des Aldehydes et des acetones a 
presence du Sulfite de Soudre dans le Developpement de L’Image Latente_ . 
Photographique. Bull. Soc. franc. Phot., 18906, , 558. om 
LUMIERE AND SEYEWETz—Sur L’Utilisation Pratique de L’Acetone comme Suc- 
cedane des Alcalis dans les Developpateurs Alcalins. Bull. Soc. franc, 
Phot., 1897,  , 550. 
LUMIERE AND SEYEWETZ—Sur L’Alteration a L’air du Sulfite de Sadr An- 
hydre. Bull. Soc. franc. Phot., 1904, , 226. 
LUMIERE AND SEYEwETz—Action of Alkalis in Organic Developers. Bull. Soci 
franc. Phot., 1906, 22, 32; Phot. J., 1906, 46, 160. - 
LUMIERE AND SEYEWETZ—Desensitizers for Plates and Papers. Brit. J. Pho 
1922, 69, 351, 370. 
Luppo-CRAMER—Desensitization by Isocyanines and Carbocyanines in Present e 
of Soluble Bromides. S. I. P., 1923, 3, 58. ae 


——— Oe a 


ee eS 


REFERENCES TO TECHNICAL JOURNALS 617 


Lupro-CraMER—Destruction of the Latent Image and Desensitization. Phot. 
Ind., 1923, p. 236. 

Luppo-CraMER—Prevention of Chemical Fog with Desensitizer. Phot. Ind., 
1924, Pp. 433. 

Luppo-CraMer—Desensitizing Colorants and their Leucobases. Phot. Ind., 
1925, p. 56. 

Luppo-CRAMER—For all papers on Desensitization see Negativ Entwicklung Bei 
Hellem Lichte, 1922. 

Luppo-CraMER—On the Watkins System of Factorial Development. Phot. 
Rund., 1921, p. 81. 

Lupro-CrAMER—Development Paradoxes. Phot. Ind., 1924, p. 6. 

MerEs AND WRATTEN—Variations in the Watkins Factor. Brit. J. Phot., 1907, 
54, 560. 

MeEES AND WratreN—Development by Time. Brit. J. Phot., 1910, 57, 376; 
Phot. J., 1910, 50, 403. ' 

Newton—A Pyro Developer for Great Contrast. Brit. J. Phot., 1916, 62, 62. 

PaTHE CINEMA (Research Laboratory)—New Desensitizers. Rev. franc. Phot., 
1924, 5, 286.. 

PaTHE CINEMA (Research Laboratory)—Oxidation Fog and Desensitizers. 
Rey. franc. Phot., 1925, 6, 33. 

Rem—A Comparison of Desensitizing Agents. Brit. J. Phot., 1925, 72, 10. 

Rosst—Decoloration of Emulsions Desensitized in Safranine. Rev. Fot. Ital., 
1923, 8, 100. 

Scottr—Preservation of Solutions of Sodium Sulphite. J. London Camera 
Club, 1923, I, 3. 

SHEPPARD AND ANDERSON—Equivalence of Sodium and Potassium Carbonates 
in Developers. Brit. J. Phot., 1925, 72, 232. 

SowErsy—Allowance for Subject in Time Development. Amat. Phot., 1915, 

» 439. 

STEIGMANN—Experiments on Desensitizers. Phot. Ind., 1923, p. 458. 

Watxins—Method and Instrument for Timing Development. Brit. J. Phot., 
1894, 41, 120, 125; Phot. News, 1804, 38, IIS. 

Wartxkins—Control over Results in Development. Phot. J., 1895, 19, 161. 

WaTKINs—Some Developers Compared. Phot. J., 1900, 24, 221. 

WatTKiIns—Some Aspects of Photographic Development. Brit. J. Phot., 1902, 
49, 1025. 

Watxkins—Developing Speed of Plates. Brit. J. Phot., 1908, 55, 382, 4or. 

WaTKINS—Time Development Calculator. Brit. J. Phot., 1908, 55, 646. 

Wartxins—Some Recent Aids to Time Development. Phot. J., 1909, 49, 367; 
Brit. J. Phot., 1900, 56, 913. 

WaTKINS—Time Development. Amat. Phot., 1910, 51, 481, 500; Brit. J. Phot., 


1910, 57, 387. 

Watxkins—tTesting the Developing Speed of Plates. Brit. J. Phot. 1921, 68, 
383. 

Watt—The Alkalis in Development. Amer. Phot., 1922, 481; Brit. J. Phot., 
1922, 69, 634. 


Wati—Development in a Bright Light. Amer. Phot., 1921, 15, 651. 


618 PHOTOGRAPHY a 


Wa.tt—Sulphites, Metabisulphites and Acid Sulphite. Amer. Phot., 1922, 16, 
137, i 


Chapter XIV. The Laws of Fixation and Washing “= 


(For list of general reference works see page 358) a 
ELtiott, SHEPPARD AND SwEET—The Chemistry of the Acid Fixing Bath. ye 
Frank. Inst., 1923, 196, 45. 
ExLspEN—The coe and Practice of Washing. Phot. J., 1917, 57, 90; Brit, 14 
Phot., 1917, 64, 120. | 
GAEDICKE—Rapid Washing of Plates. Phot. Woch., 1906, p. 41. 
HicKMAN AND SPENCER—Washing of Photographic Products. Phot. J., 
62, 225; 1923, 63, 208. Brit. J. Phot., 1922, 69, 387, 400. * 
HIcKMAN AND SPENCER—The Washing of Photographic Products, Parts Iv, 
V,°ViL= Phot; Js ro24) GAnss0. ; 
LUMIERE AND SEYEWETZ—Action des Alums et des Sels d’Alumine sur la Gelawd 
tine. Bull. Soc. franc. Phot., 1906, —, 267. q 
LUMIERE AND SEYEWETZ—Sur L Tnsalublieaeee de la Couche Gelatinee des 
Plaques ou des Papiers Photographiques dans le Bain de Fixage. Bull. ~ 
Soc. franc. Phot., 1906, —, 306. a 
LUMIERE AND SEYEWETZ—Sur L’Insolubilisation de la Gelatine par Formalde- § 
hyde. Bull. Soc. franc. Phot., 1906, —, 364. 
LUMIERE AND SEYEWETZ—Sur la inate D’Emploi des Bains de Pica. Bull, 
Soc. franc. Phot., 1907, —, 10. 4 
LUMIERE AND SEYEWETZ—Sur L’Emploi de L’Hyposulfite, D’Ammoniaque a 
D’un Melange D’Hyposulfite de Soude et D’un Sel Ammoniacal pour 
le Fixage des Plaques et des Papiers. Bull. Soc. franc. Phot., 1908, —, 
217. + 
LUMIERE AND SEYEweTz—Sur L’Elimination par Lavage a L’eau de L’Hypoa 
sulfite de Soude Retenu par les Papiers et les dbigci Photographiques. 4 
Bull. Soc. franc. Phot., 1902, —, 251. 7 
LUMIERE AND Sivewerthe Time ak Fixing of Developing Papers. Brit. 
J. Phot., 1924, 7x, 108. Bull. Soc. francs Phot; 16247 pan a 
LUMIERE AND SEYEWETZ—The Fixing of Photographic Negatives. Rev. franc. 
Phot., 1924, 5, 61. a 
LUMIERE AND SEYEWETZ—The Rapid Washing of Photographic Negatives. 
Rev. franc. Phot., 1922, 3, 109. 4 
LUMIERE AND SEYEWETZ—Fixing in Sodium Thiosulphate with the eee of 
Ammonium Chloride. Rev. franc. Phot., 1924, 5, 204. ae 
Piper—The Rate of Fixing. Brit. J. Phot., 1913, 60, 50. ; ae 
PiperR—Rapid Fixing Baths. Brit. J. Phot., 1914, 61, 193, 437, 458, 511. 4 
PipeErR—Further Experiments on Fixing. Brit. J. Phot., 1915, 62, 364. 
SHEPPARD AND Mres—Theory of Fixation. Phot. J., 1906, 46, 235. 
Warwick—Scientific Washing of Negatives and Prints. Amer. Phot., 17, 4 


II, 317. 


PAPERS ON THE LAWS OF FIXING AND WASHING 


i 


Warwick—The Laws of Fixation. Amer. Phot., 1917, 11, 585. a 
Warwick—The Fixation of Prints. Amer. Phot., 1917, 11, 639. a 
" oy 

vie - 


~, oe —, 


ee ee ae 


REFERENCES TO TECHNICAL JOURNALS 619 


Chapter XVI. Intensification and Reduction 
(For list of general reference works see page 389) 


REDUCTION 


ANDRESEN—Hydrogen Peroxide as a Reducer. Phot. Korr., 1890, 36, 256. 

BacHrRAcH—The Mercury-Cyanide Reducer. Brit. J. Phot., 1916, 63, 163. 

BayLtEy—Persulphate and Sulphocyanide Reducer. Phot. News, 1900, 44, 174. 

BENNETT—Ammonium Persulphate Reduction. Phot. J., 1907, 47, 328. 

BoTHAMLEY—Some Minor Processes in Photography. Phot. J., 1918, 58, 48. 

DrEBENHAM—The Hypochlorite Reducer. Brit. J. Phot., 1916, 63, 487, 538. 

Drecx—The Permanganate-Persulphate Reducer. Brit. J. Phot., 1916, 63, 301. 

Dopcson—Notes on the Action of Ammonium Persulphate as a Reducer. Phot. 
daetoit, St. 205, 302: Brit. J. Phot., 1911, 58, 503, 742. 

Hetain—The Theory of Persulphate Reduction. Bull. Soc. franc. Phot., 1899, 
15, 304. 

Hicson—Reaction between the Persulphates and Silver. J. Chem. Soc. (Lon- 
don), 1921, 119, 2048. 

Hicson—History of Persulphate Reduction. Phot. J., 1921, 61, 237. (Full 
bibliography. ) 

Hicson—Potassium Persulphate as a Reducer. Phot. J., 1922, 62, 08. 

Huse Aanp Nerrz—Proportional Reducers. Brit. J. Phot., 1916, 63, 580. 

Huse anp Neirz—The Hypochlorite Reducer. Brit. J. Phot., 1917, 64, 143. 

LUMIERE AND SEYEWETZ—The Action of Persulphate of Ammonia on Metallic 
Silver. Brit. J. Phot., 1808, 45, 473. 

LUMIERE AND SEYEWETZ—The Theory of Persulphate Reduction. Bull. Soc. 
franc. Phot., 1800, 15, 226. 

J.UMIERE AND SEYEWETZ—Reducers. Brit. J. Phot., 1900, 47, 805. 

LUMIERE AND SEYEWrTz—On the Irregularities in the Action of the Persulphate 
Reducer. Brit. J. Phot., 1921, 68, 124. 

Luppo-CraMER—The Chemistry of Persulphate Reduction. Brit. J. Phot., 1901, 
48, 89; Phot. Korr., 1901, 38, 17. 

Lupro-CRAMER—The Action of Reducers and its Dependence on the Constitution 
of the Image. Eder’s Jahrb., 1906, 20, 237. 

Lupro-CRAMER—The Composition of Negative Substances and its Influence on 
Reduction. Phot. Korr., 1907, 54, 940. 

Lupro-CRAMER—The Action of Reducers. Phot. Korr., 1907, 54, 230. 

Luppo-CRAMER—Absorption Complexes in the Silver Grain as the Cause of the 

; Persulphate Effect. Phot. Korr., 1908, 45, 159. . 

Lupro-CRAMER—Reduction with Oxidizers containing Halides and with Per- 
sulphate. Phot. Korr., 1910, 47, 489; I911, 48, 466. 

Lurro-CRAMER—The Dispersoid Theory of Persulphate Reduction. Phot. Korr., 
IQI2, 49, 118. 

Lupro-CRAMER—The Theory of Persulphate Reduction. Phot. Korr., 1914, 51, 
240. 

Namias—Ammonium Persulphate Reduction. Phot. Korr., 1890, 36, 86, 144, 
216, 


620 PHOTOGRAPHY 


Namras—The Use of Ammonium Persulphate. Eder’s Jahrb., 1901, 15, 165. 

Namias—A Comparative Study of Photographic Reducers. Il. Prog. Fot., 
1922, 29, 161. 

Patmer—A Copper Bromide Reducer for Decreasing Contrast. Phot., 1915, 
Pp. 420. 

Pinnow—Reduction with Persulphate. Zeit. Wiss. Phot., 1908, 6, 130. 

Puppy—The Sulphocyanide-Persulphate Reducer. Phot., 1900, p. 99. 

ScuErrer—Researches on the Action of Reducers. Brit. J. Phot., 1908, 55, 472. 

ScHULLER—The Theory and Practice of Reduction. Phot. Rund., 1910, 24, 113, 
161. 


ScHULLER—Persulphate Reduction. Eder’s Jahrb., 1913, 27, 419; Phot. Rund., — 


1912, 26, 270. 
SHEpparp—The Effect of the Iron Content of Ammonium Persulphate on its 
Photographic Reducing Power. Brit. J. Phot., 1918, 65, 314. 
Syepparp—The Action of Soluble Chlorides and Bromides on Reduction with 
Ammonium Persulphate. Phot. J., 1922, 62, 321. 
SuHEepparpD—Persulphate Reduction. Phot. J., 1921, 61, 450. 
SmitH—The Cobaltine Reducer. Brit. J. Phot., 1914, 61, 59. 
STENGER AND HELLER—Reduction with Persulphate. Zeit. Wiss. Phot., 1911, 9, 


73- 


STENGER AND He_tER—The Persulphate Reducer. Zeit. f. Reproductionstechnik., — 


IQIO, 12, 162, 178; I9II, 13, 5, 20, 34, 50, 70, 84, 100. 


STENGER AND HeLter—Reduction with Persulphate. Part Il. Zeit. Wiss. Phot., : 


IQII, 9, 389. 


STENGER AND Hetter—Reduction with Persulphate. Part III. Zeit. Wiss. Phot. — 


1913, 12, 300. 


STENGER AND HELLER—Reduction with Persulphate. Part IV. Zeit. Wiss. Phot. 


1915, 14, 177. 


SrieGMANN—Persulphate Effect with a Bleacher of Mercury and Copper. Phot. a 


Rund., 1921, 4, 52. 


ST1EGMANN—Mercuric Nitrate and Sulphate as Proportional Reducers. Phot. F 


Ind., 1921, p. 697. 


Wusey—Intensification and Reduction with Pyro Developers. Brit. J. Phot, @ 


1910, 66, 721. 


_____ Softening Contrast by Re-Development. Brit. J. Phot. 1914, 61, 788. — 


INTENSIFICATION 


Baxer—The Theory and Practice of Intensification. Brit. J. Phot., 1906, 53, 


264, 284, 309. 


CatLier—Powerful Intensification of Gelatine Plates. Brit. J. Phot., 1911, 58, 7 


452. 
Cuartes—A Bichromate-Mercury Intensifier. Brit. J. Phot. 1919, 66, 172. 


CLErRc—Desalme-Intensification with Copper and Tin. Brit. J. Phot., 1912, 59, 3 


215, 266; Bull. Soc. franc. Phot., pp. 96, 99. 


CrowTHER—Chromium Intensification with Chlorochromate. Brit. J. Phot. — 


1919, 66, 709. 
CuNNINGHAM—Intensification. Brit. J. Phot., 1915, 62, 818. 


ee ee 


To 


eS a ee Te ee 


Cg Ee ge OR ee 


REFERENCES TO TECHNICAL JOURNALS 621 


Eper—Modern Intensifiers for Gelatino-Bromide Plates and their Effects. Brit. 
J. Phot., 1900, 47, 68; Phot. Korr., 1900, 37, 23. 

Eper—Fffect of Intensification. Brit. J. Phot., 1900, 47, 460. 

Ives—Intensification by Dye Toning. Brit. J. Phot., 1921, 68, 187. 

JaNKo—A Comparative Table of the Effects of Various Intensifiers. Brit. J. 
Phot., 1900, 47, 518. 

Jones—Intensification with Mercuric Chloride and Ferrous Oxalate. Phot. J., 
IQIO, 50, 238. 

Jones—On the Proposed Substitutes for the Ferrous Oxalate Developer. Phot. 
J., 1910, 50, 242. 

LUMIERE AND SEYEWETZ—Sur L’emploi de L’iodure Merique comme Renforca- 
teur. Bull. Soc. franc. Phot., 1890, p. 472. 

LUMIERE AND SEYEWETZ—Sur L’emploi des Quinones et de leurs derives Sul- 
foniques pour Renforcer les Images Argentiques et pour les Virer en Dif- 
ferentes Couleurs. Bull. Soc. franc. Phot., 1910, p. 360. Brit. J. Phot., 
1910, 57, 949; I9II, 58, 460. 

LUMIERE AND SEYEWETZ—Intensification with Salts of Chlorochromic Acid. 
Phot. Korr., 1920, 57, 282. 

LUMIERE AND SEYEWETZ—Toning and Intensification with Toluquinone. Rev. 
fr. Phot., 1922, 3, 203. 

Namias—The Mercuric Iodide Intensifier. Il. Prog. Fot., 1921, p. 103. 

NamiAs—Extreme Intensification. Brit. J. Phot., 1922, 69, 149. 

Neitz anD Huse—The Sensitometry of Photographic Intensification. Phot. J., 
1918, 58, 81. 

Pirper—Chromium Intensifiers. Brit. J. Phot., 1907, 54, 3 

Piper—Intensification by Increase of the Bulk of the Image Compound. Brit. 
J. Phot., 1908, 55, 195. 

Pirper—Physical Intensification with Mercury. Brit. J. Phot., 1916, 63, 1, 67. 

SmirH—Silver Intensification. Brit. J. Phot., 1909, 56, 82. 

Witsey—Intensifying by Redevelopment with Pyro. Brit. J. Phot., 1919, 66, 
721. 

WELLINGTon—Intensification with Silver. Brit. J. Phot., 1911, 58, 551. 

Intensification. Brit. J. Phot., 1915, 62, 570. 

Intensification by Re-Development. Brit. J. Phot., 1915, 62, 426. 


Chapter XVII. Printing Processes with Silver Salts 


(For list of general reference works see page 414) 


THE SENSITOMETRY OF SILVER DEVELOPMENT PAPERS 


BLAcKsTRoM—Sensitometry of Photographic Papers. Nord. Tids. Fot., 1922, 
6, 121. 

ForRMSTECHER—Absolute Gradation as a Characteristic Ganstant of Photo- 
graphic Papers. Zeit. wiss. Phot., 1922, 21, 21. 

GLover—Experiments with Bromide and Gaslight Papers. Brit. J. Phot., 1920 
67, 139, 151, 169. 

GLover—Contrast Rating of Gaslight and Fromide Papers... Phot, ‘J.,.1022, 62, 
132; Brit. J. Phot., 1922, 69, 156. 
41 


622 PHOTOGRAPHY 


Goopwin—Capacity of Printing Processes for Rendering Gradation. Brit. = 
Phot., 1909, 52, 187, 207, 227. 
HENDERSON—Speed and Gradation of Papers. Brit. J. Phot., 1916, 63, 311. 


HurTER AND DriFFIELD—Relation between Photographic Negatives and their 


Positives. J. Soc. Chem. Ind., 1891, 10, 100; Eder’s Jahrb., 1893, 7, 18; 
H. and D. Memorial Volume. 
Jones, NutTINc anpD MrEes—Sensitometry of Photographic Papers. Phot. J., 
1914, 54, 301; Brit. J. Phot., 1915, 62, 9, 22, 38. 

JonEs AND Firt1us—The Gloss Characteristics of Photographic Papers. Brit. 
J. Phot., 1922, 69, 216, 220. 

OpENcRANTS—The Investigation of Development Papers. Nord.. Tids. Fot., 
1922, 6, 70. 

RENwick—The Sensitometry of Photographic Papers. Phot. J., 1915, 55, 20. 


THE HANDLING OF DEVELOPMENT PAPERS 


BarnEes—Glazing Prints. Brit. J. Phot., 1923, 70, 798. 

BrowNn—Practical Notes on Printing Processes. British Journal Almanac, 
IQ16, p. 342. 

Davis—Controlling Tone Values by Compensating Positives. Phot. Era, 1921, 
p. 231. 

GLoverR—The Case for the Factorial Development of Bromide Paper. Brit. J. 
Phot., 1921, 68, 503, 519. 

GLOvER—The Development of Gaslight Papers. Amer. Phot., 1923, 17, 20. 

GLoveErR—The Development of Bromide Paper Prints. New Photographer, 
1923, p. 64. 

Kruc—Just Plain Prints. Amer. Phot., 1922, 16, 60. 

Jones AND FawxKes—Sensitometric Study of the Reduction of D-O-P Paper 
Prints. Brit. J. Phot., 1921, 68, 275. 


JoNES AND CRABTREE—A New Densitometer for Determining the Time of es . 


posure in Positive Printing. J. Soc. Mot. Pict. Eng., 1923, p. 89. 
Jorpan—Still Another M-Q Developer for Gaslight Papers. Amer. Phot., 
1923, 17, 139. 
LAMBERT—A Consideration of the Technical and Artistic Qualities of Printing 
Processes. Phot. J., 1924, 64, 266. 


Mayer—The “ Drem” Exposure Meter (for positive printing). Phot. Rund., 


1924, 61, 12. 


Chapter XVIII. Projection Printing 


(For list of general reference works see page 444) 


Bani—Positives Direct on Bromide Paper. Phot. Journ. of Amer., 1922, 60, 
440. 

BRAWTREE—Positives by Reversal on Dry Plates. Brit. J. Phot., 1914, 61, 320. 

Canpy—The Best Lighting System for the Amateur’s Enlarger. Amer. Phot., 
1919, 13, 200. 


aS at oe ee eet 
OO ee. ee ee 


~~ 


& 


ea ee 


. 
| 
| 


REFERENCES TO TECHNICAL JOURNALS 623 


Canpy—Selection, Application and Manipulation of Condensing Lenses for Pro- 
jection Printing. Amer. Phot., 1923, 17, 588. 

CuHaArLEsS—Enlarging without Condensers. Brit. J. Phot., 1921, 68, 600. 

Cottins—Exposure, Scale, Aperture and Distance in Lantern Reproduction. 

_ Brit. J. Phot., 1923, 70, 31. 

Cousms—Illuminating Factors in Pareie: Brit. J. Phot., 1917, 64, 16. 

DrirrigLp—The Principles Involved in Enlarging. Brit. J. Phot., 1804, ay 714, 
721; H. and D. Memorial Volume. 

oF ee reneiain-— Making a Parabolic Illuminator for Enlarging. Phot. Era, 1915, 
p. 66. 

Frary, MitcHELL AND BAKER—Positives Direct by Reversal. J. Soc. Chem. 
Ind., 1912, p. 901; Brit. J. Phot., 1912, 59, 788. 

GaILLArD—A Vertical Enlarger. Brit. J. Phot., 1915, 62, 812. 

GumBEert—Positives Direct with Thiocarbamide. Amer. Phot., 1915, 8, 124; 
Brit. J. Phot., 1915, 62, 167. 

HENDERSON—Finding Exposures in Bromide Enlarging. Brit. J. Phot., 1915, 62, 
448. 

Jacoss—Exposure in Artificial Light Enlarging. Amer. Phot., 1921, 15, 490. 

Kinc—Calculation of Exposures in Enlarging. Brit. J. Phot., 1906, 53, 188. 

Kruc—Speeding up the Enlarger. Amer. Phot., 1923, 17, 453. 

Locxett—The Calculation of Exposures in Danio Halereine. Hrit..J.-F not. 

1905, 52, 845. 

Lockett—Enlarging to Scale. Brit. J. Phot., 1917, 63, 350. 

Locxett—A Suggested Type of Enlarging Lantern. Brit. J. Phot., 1918, 66, 393. 

Locxett—Enlarging to Scale with Supplementary Lenses. Brit. J. Phot., 1920, 


a. . 07, $71. ; 
Locxett—A Self-Focussing Vertical Enlarger. Brit. J. Phot., 1923, 70, 760. 


‘Locxetr—A Focussing Scale for Enlarging. Brit. J. Phot., 1924, 71, 171. 


MarsHALL—A Vertical Enlarger for Artificial Light. Brit. J. Phot., 1917, 64, 
160. : 

Moyne—Enlarging Easel. British Patent 124,639/1918; Brit. J. Phot., 1919, 66, 
325. 

2 ‘Pica ” —Correction.,of Distortion when Enlarging. MHarrington’s Photographic 
Journal, 1916, p. 323. 

Piper—Correction of Distortion Produced by Tilting Camera. Brit. J. Phot., 
1908, 55, 604. | 

SELLors—Method of Calculating Exposures in Enlarging. Brit. J. Phot., 1923, 
79, 349. 

“ Tuermit ”—An Easel for Rapid Enlarging. Brit. J. Phot., 1923, 70, 36. 

THompson—A Portable Enlarger for Gaslight Papers. ee Phot., 1913, 7, 
142; 1914, 8, 150. 

THomson—A Vertical Enlarger. Brit. J. Phot., 1921, 68, 746. 

Youne—Portable Enlarging Apparatus. Camera Craft, 1921, p. 161. 


624 PHOTOGRAPHY 


Chapter XIX. The Lantern Slide 


(For list of general reference works see page si 


Acra—Quinone Bleach for Dye Toning. British Patent 180,202; Brit. ae Phot 
1922, 69, 426. v 


77: 
BenNnetr—Lantern Slides Direct by Reversal. Amat. Phot., rort, p. 55. 
BrowNn—Lantern Slide Making. British Journal Almanac, 1912, p. 405. 
CuHarLEs—Slide Making Attachment for the Enlarger. Brit. J. Phot., — 69, qi 


232. . 
GLover—Factorial Development for Lantern Slides. Brit. J. Phot, —_ 67 
Grovea‘Thictarhamide and Blue Toned Lantern Slides. Brit. J. Phot, 192 

7°, 135. a << 


GreENALL—Control in Lantern Slide Making. Phot., 1916, p. 118. 
Ives—Dye Toning. Brit. J. Phot. (color supplement), roro, 66, 1. 


186; 1921, 68 foolar odcoienene mi 
JoHNson—Personal Practice in Lantern Slide Making. Phot. J., r918, oe 
JouNson—The Technics of Lantern Slide Making. Brit. J. Phot., 19RD Bs 

237; Phot. J» 1923, 63, 58. 


Phot. J., 1911, 51, 159—first paper; Phot. J., 1917, 57, 138—correction « ol 
first paper. eh 
Kettey—Copper Bichromate Bleach for Dye Toning. British. Patea, 
137/1921; Brit. J. Phot., 1922, 69, 330. ee 
Power—Lantern Slides by Reversal. Brit. J. Phot., rort, 58, 104, 
Powrer—Dye Toning of Lantern Slides. Brit. J. Phot., 1912, 59, 503. . 
Powrer—Dye Toning. Brit. J. Phot. 1912, 59, 41. 
Rosach—New Method of Dye Toning. Brit. J. Phot., 1923, 70, 363. i 
Trause—Copper Morgass Process. British is 163387 /1o8; ae " P 


British Patent 163,336, 163,337; Brit. J. Phot. ste 68. “(olor supp 
ment), 32. 2 

Wirson—Dye Toning. Brit. J. Phot., 1912, 59, 503. Be hike 

Lantern Slides (Decennia Praction): Brit. J. Phot., 1916, 63, su, 


ee eS 
23) Seal ea. 


REFERENCES TO TECHNICAL JOURNALS 625 


Chapter XX. The Toning of Developed Silver Images 
(For list of general reference works see page 478) 
Sepia Toning by the Hypo-Alum Process 


DrINKWATER—Sulphur Toning. Brit. J. Phot., 1923, 70, 204. 

E. K. Company—Hypo-alum-gold Toning Bath. Phot. Era, 1911, p. 258. 

SEDERQUIST—Gold in the Hypo-Alum Toning Bath. Brit. J. Phot. 1920, 67, 
437. 

THEermMit—aAccelerated Hypo-alum Toning. Brit, J. Phot., 1922, 69, 126. 

Ripening with Ammonia. Brit. J. Phot., 1922, 69, 126. 

Hypo-alum-gold Toning Bath. Brit, J. Phot., 1921, 68, 650. 


Toning with “ Liver of Sulphur” and the Polysulphides 


Butirocx—Polysulphide Toning. Brit. J. Phot., 1921, 68, 393. 

Fenske—Liver of Sulphur Toning, B. P. 18545 of 1012. 

LUMIERE AND SEYEWETZ—Sulphuration Directe des Image Argentique sur 
Papier au Moyen du Foie de Soufre. Bull. Soc. france. Phot., 1923, p. 
320. 

UnperserG—Toning with the Polysulphides. Brit. J. Phot., 1924, 71, 50. 

Vero—Liver of Sulphur Toning. Brit. J. Phot. 1912, 59, 774. 

WoopMan—Liver of Sulphur Toning. Brit. J. Phot., 1912, 59, 565. 

Raw.iines—Liver of Sulphur Toning. Brit. J. Phot., 1914, 61, 218. 

Liver of Sulphur Toning. Brit. J. Phot., 1916, 63, 505, 606. 


Toning by the Indirect Sulphiding Process 


Atitport—An Iodine Bleacher for Sulphide Toning. Amat. Phot. (London), 
1923, 55, 407. 

Baker—Factors in Sulphide Toning. Brit. J. Phot., 1912, 59, 609, 

Bu._tock—Experiments in Sulphide Toning. Brit. J. Phot., 1921, 68, 442, 447. 

Baxer—Non-Bromide Bleach for Sulphide Toning. Brit. J. Phot., 1916, 63, 
626. - 

Carngecir—The Chemistry of the Sulphide Toning Process. British Journal 
Almanac, 1907, p. 676. 

Goutpinc—The Quinone Bleacher. Brit. J. Phot., 1915, 62, 725. 

GREENALL—A Phosphate Ferricyanide Bleacher for Sulphide Toning. Phot., 
IQI2, p. OI. 

GreenaALL—A Non-Acid Permanganate Bleacher for Sulphide Toning. Brit. J. 
Phot., 1916, 63, 621; Brit. J. Phot. 1917, 64, 371, 382. 

Greenatt—Acid Bleachers for Sulphide Toning. Brit. J. Phot., 1917, 64, 30. 

HrrMANSoN—Range of Tones in Indirect Sulphide Toning. Brit. J. Phot., 
1916, 63, 626. 

Lumitre aND Sryewetz—Sulphiding with Sulphoxyphosphate. Rev. france. 
Phot., 1921, Supp. 4. 

LuMI&RE AND SEYEWETZ—Toning Red with Silver Sulphide. Rev. franc. Phot., 


1923, P. 133. 


626 PHOTOGRAPHY 


LUMIERE AND SEYEWETZ—Toning with Quinone. Brit. J. Phot., 1921, 68, 6. 


Namias—Barium Sulphide for Sulphiding. P. Mitt, roz1, 7, 100; Brit. J. 


Phot., 1911, 58, 324. 
Punnett—Sulphocyanide-sulphide Toning. Amer. Phot., 1907, p. 25. 
SmitH—Bleaching of Sulphide Toned Prints. Brit. J. Phot., 1914, 61, 402. 
SmitH—Reducing Sepia Toned Prints. Phot. J., 1907, 47, 281; Brit. J. Phot., 
1907, 54, 595. 
Strauss—Contribution to Sulphide Toning. P. Ind., 1924, p. 78. 
THoMSoN—Sepia Tones by the Sulphide Process. Amer. Phot., 1921, 15, 610. 


Miscellaneous Processes of Sulphur Toning 


BLAKE-SMITH—Single Solution Sulphide Toner. Brit. J. Phot., 1911, 58, 140. 

Kropr—Single Solution Sulphide Toner. Brit. J. Phot., 1910, 57, 836; Phot. 
Rund., 1910, 21,:245. 

PuNNEtTT—Single Solution Sulphide Toner with Ammonium Sulphocyanide. 
Brit. J. Phot., 1910, 57, 860. 


SHAw—A New Method of Cold Sulphide Toning. Brit. J. Phot., 1923, 70, 267. — 


SHAw—The Theory of Nitro-Sulphide Toning. (In reply to Sheppard.) Brit. 
J. Phot., 1923, 70, 591. 

SHAw—An Improved Method of Single Sotutian Cold Sulphide Toning. Brit. 
J. Phot., 1923, 70, 750. 

SHEPPARD—The Theory of Toning with Nitro-Suiphide Bodies. Brit. J. Phot., 


1923, 70, 547. 


TriePeEL—Cold Single Solution Sulphide Toner. B. P. 24,378 of 1910; Brit. J. 


Phot., 1911, 58, 657. 
VALENTA—Single Solution Sulphide Toning. Brit. J. Phot., 1912, 59, 313. 


Journ. Almanac, 1916. 


Toning with Copper, Uranium and Iron 


CosENzL—Iron, Copper and Uranium Toning Processes. Phot. Korr., 1922, 59 
(Festnummer), p. II. . 
GREENALL—Intensified Copper Toning. Amat. Phot., 1919, p. 27. 
Lupro-CrRAMER—Clearing the Whites of Images Toned with Uranium and Iron. 
Camera (Luzern), 1923, 2, 177. _ . 
MurpHy—Copper-Tin Toning. Amat. Phot., 1922, p. 547. 


Namias—Copper Toning. P. Korr., 1907, p. 229; Brit. J. Phot., 1907, 54, 303. ; 


Namias—lIron and Vanadium Toning. Il Prog. Fot., 1922, 29, 85. 


SEDLACZEK—Ferricyanide Toning. (Uranium.) P. Ind., 1924, pp. 205, 234; 


Amer. Phot., 1924, p. 4. 

_ Srrauss—Toning with Copper. Phot. Rund., 1922, 59, 147. 
Strauss—Copper-Chromium Toning. Brit. Journ. Almanac, 1923, p. 367. 
THomson—Uranium as a Toner and Intensifier. Amer. Phot., 1920, 14, 648. 


Warp—Copper-Sulphide Toning. B. P. 8002 of 1912; Brit. Journ. Almanac, — 


1914, p. 659; B. P. 6026 of 1913; Brit. Journ. Almanac, 1914, p. 423. 


Sulphur Toning in an Acid Solution. Phot. Era, 1915, p. 127; Brit. 


— oT) hme) ee ee 


. 


REFERENCES TO TECHNICAL JOURNALS 627 


Toning with Cobalt, Tin and Vanadium 


Druce—Toning with Tin. Phot. J. of America, 1922, 60, 355; Brit. J. Phot., 
1922, 69, 433. 

FoRMSTECHER—Toning with Stannous Compounds. P. Rund., 1921, p. 277; 
Brit. J. Phot., 1921, 68, 750. 

LAMBERT—Toning with Vanadium. Brit. Journ. Almanac, 1923, p. 666. 

MurpHy—tTin and Copper Toning. Amat. Phot., 1922, p. 547. 

NamiAs—Toning with Vanadium. Rev. franc. Phot., 1924, 5, 76. 

Namias—Toning with Iron and Vanadium. II Prog. Fot., 1922, 29, 85. 

RicHARDSON—Toning with Stannous Compounds. Amat. Phot., 1923, 55, 469. 

SOMERVILLE—Toning with Vanadium. Photogram, 1906, p. 265. 

StRAUSS—Toning with Cobalt. P. Rund., 1923, 60, 69; Brit. J. Phot., 1923, 70, 
352. 

StTrAuss—Toning with Cobalt. P. Ind., 1924, p. 232. 

Watit—Toning with Vanadium. Phot. Journ. America, 1921, 59, 96. 


Miscellaneous Toning Processes 


FoRMSTECHER—Toning with Selenium. B. P. 169,378/1920; Brit. J. Phot., 1921 
68, 650. 

FoRMSTECHER—Toning with Palladium. P. Ind., 1922, p. 774. 

ForstMANN—Two Color Tones with Selenium. Brit. J. Phot., 1921, 68, 410. 

Gaupet—Toning with Colloidal Silver. French Patent 514,016. 

Mimosa Axkt.—Toning with Cadmium and Mercury. B. P. 130,517; Brit. J. 
Phot., 1920, 67, 290. 

NaAmiAs—Sulpho-Selenium Toning. Brit. J. Phot., 1920, 67, 648. 

NamiAs—Sulpho-Selenium Toning. Il Prog. Fot., 1922, 29, 203. 

RAWLING—Toning with Colloidal Sulphur. Phot. J., 1922, 62, 3. 

SEDLACZEK—Toning with Mercury. Brit. J. Phot., 1906, 53, 624, 645. 

STEIGMANN—Mercury Toning by the Orywall Process. P. Ind., 1921, p. 797. 

STEIGMANN—Toning with Sodium Hydrosulphite. P. Ind., 1924, p. 649; Sci. 
et Ind. P., 1924, 4, 75. 

Watit—Selenium Toning. Amer. Phot., 1922, p..55. 


Chapter XXI. Platinotype and Iron Printing Process 


(For list of general reference works see page 488) 


ANpDERSON—The Choice of a Printing Paper with Special Reference to Platinum. 
Amer. Phot., 1913, 7, 336, 384. 2 

Brown—Practical Notes on Printing Processes. British Journal Almanac, 
1916, p. 343. 

Burtan—An Iron-Cobalt Printing Paper. Atelier, 1921, 28, 42; Phot. J. of 
Amer., 1922, 60, 318. 

Hawxs—The Kallitype Process. Brit. J. Phot., 1916, 63, 415. 

Jacospy—On the Use of Japine Platinotype Paper. Brit. J. Phot., 1906, 53, 807. 


628 PHOTOGRAPHY 


Jacopy—A Sepia Platinum Paper. Phot. Korr., 1922, 59, 31. 

Lre1icHtoN—A Silver, Iron, Mercury Printing Paper. British Patent No. 11,- 
610/1910; Brit. J. Phot., 1911, 58, 502. 

ScHwARz—Silver-Iron Sensitizer. Brit. J. Phot., 1922, 69, 219; British Patent 
175,317/1920. 

SmitH—Modifications Produced by Variations in Strength and Temperature — 
of Developer. Phot. J., 1911, 51, 3. 

SmitH—Palladiotype. Brit. J. Phot., 1917, 64, 60, 334. 

SmMitH—Satista Paper. Brit. J. Phot., 1914, 61, 808. 

THomson—Kallitype. Amer. Phot., 1923, 17, 422. 

THomson—A Silver-Platinum Printing Paper. Amer. Phot., 1915, 9, 630. 

TuHomson—Possible Substitutes for the Platinum Print. Amer. Phot., 1917, 
I1, 642. | 

VALENTA—Ferro-Prussiate Sensitizer. Brit. J. Phot., 1917, 64, 70. : 

VALENTA—Kallitype. (An excellent summary of previously published papers 
on the subject.) Bibliography. Das Atelier, 1920, 27, 10. 

Watt—The Iron Salts. A summary of iron printing methods. Amer. Phot., 

1922, 16, 677, 766; 19023, 17, 4. : 

Recovering Platinum from Waste Baths. Brit. J. Phot.; 1920, 67, 393. 


Chapter XXII. Printing Processes Employing Bichromated — 
Colloids, I. (Carbon and Carbro) 


(For list of general reference works see page 510) 


BrenneTt—Some Improvements in Sensitizing Carbon Tissue. Phot. J., 1904, 
44, 7: f ‘ ' ; 
BraHAM—The Carbro Process. Phot. J., 1922, 62, 16; Brit. J. Phot., 1922, 69, 4. 
CarRANzA—A Quick Drying Sensitizer for Carbon Tissue. Brit. J. Phot. (col. 
supp.), 1914, 61, 3. pies 
Cuerrit—Multiple Carbon Printing. Phot., 1906, p. 327. ; 
FARMER—The Carbro Process. Amat. Phot., 1919, p. 285; Brit. J. Phot., 1910, 
66, 583; Amer. Phot., 1920, 14, 92. 
FEeLLEos—Decorative Application of Carbon Printing. (Method of Preparing 
Carbon Tissue.) Brit. J. Phot., 1920, 67, 481. 
Garon—Revised Formule for Carbro. Brit. J. Phot., 1921, 68, 327. 
Hat~t—Control in Carbro Printing. Brit. J. Phot., 1922, 69, 783. 
Harris—Tank Development of Carbon Prints. Brit. J. Phot., 1914, 61, .214. 
LUMIERE AND SEYEWETZ—Sur la Composition de la Gelatine Insolubilisee par 
les Sels de Sesquioxyde de Chrome et Theorie de L’action de la Lumiére 
sur la Gelatine Additionee de Chromates. Bull. Soc. franc. Phot., 1904, 
» 73 
LUMIERE AND SEYEWETZ—Sur la Composition de la Gelatine Impregnee de 
Bichromate de Potassium Insolubilisee par Lumiére et sur la Theorie de 
cette Insolubilisation. Bull. Soc. franc. Phot., 1905, , 440. a 
LUMIERE AND SEYEWETZ—Sur la Composition de la Gelatine Insolubilisee par 
la Lumiére en Presence de L’acid Chromique et des Principaux Bi- 
chromates Metalliques. Bull. Soc. franc. Phot., 1905, , 461. 


REFERENCES TO TECHNICAL JOURNALS 629 


LUMIERE AND SEYEWETZ—Sur la Composition de la Gelatine Bichromatie In- 

solubilisee Spontanement dans L’Obscurite. Bull. Soc. franc. Phot., 1905, 
» 541. 

‘MippLETON—Some Experiments and Notes on Pictures in Pigments. Brit. J. 
Phot., 1923, 70, 735. 

Namras—Reaction of Various Compounds of Chromium with Gelatine. Phot. 
J., 1902, 42, 195. 

PretrascH—The Development of Over Exposed Carbon Prints. Phot. Rund., 
1912, p. 57; Brit. J. Phot., 1912, 59, 217. 

Watit—The Chromium Salts. Amer. Phot., 1922, 16, 613. 

Wati_—Substratum for Carbon Transparencies. Brit. J. Phot., 1914, 61, 459. 

Watit—The Carbon Process. Amer. Phot., 1924, 18, 1, 86. 

WarsurcG—Dyes as Sensitizers of Carbon Tissue. Phot. J., 1917, 57, 160. 


Chapter XXIII. Printing Processes Employing Bichromated 
Colloids, II. (Gum-Bichromate and Allied Processes) 


(For list of general'reference works see page 523) 


ANDERSON—The Gum Pigment Process. Amer. Phot., 1913, 7, 504, 584, 648, 
700, 707; 1914, 8, 8, 12, 76. 

ANDERSON—Multiple Gum Printing. Amer. Phot., 1912, 6, 676. 

Battry—A Simplified Method of Printing in the Gum-Bichromate Process. 
Phot. J., 1923, 63, 308. | 

Davis—Gum-Bromide Printing. Amer. Phot., 1921, 15, 53. 

GRANDMAITRE—Multi-Layer Gum Process. Bull. Soc. franc. Phot., 1923, 10, 16. 

Kruc—Gum Printing. 

LetcHton—A Method of Working the Gum-Bichromate Process. American 
Annual of Photography, 1924, p. 40. 

Lrspy—Multiple Gum. American Annual of Piawseriniy, 1922, p. 124. 

MacnaMARA—Multiple Gum. Brit. J. Phot., 1919, 66, 320. 

Mente—Glue Printing. Camera (Luzern), 1922, I, 144. 

MorrpyKE—Multiple Gum Process. Camera Craft, 1921, p. 308. 

Owen—A Gum Printing Frame. Amer. Phot., 1923, 17, 416. 

RicHER—The Glue Print. Amer. Phot., 1923, 17, 38. 

STARNES—The Gum-Bichromate Process and a New Colloid. Phot. J., 1918, 
58, 287; Brit. J. Phot., 1919, 66, 5o. 

ZeERBE—Method of Registration for Multiple Printing. Camera Craft, 1923, p 
214; American Annual of Photography, 1923. 

ZERBE—The Gum-Platinum Process. Amer. Phot., 1910, April. 


Chapter XXV. Copying 


BraAMWELL—A Copying and Enlarging Cabinet. British Patent 155,906/I1919; 
Brit. J. Phot., 1921, 68, 142. 

BraMwett—Focussing Enlarged Copies. Brit. J. Phot., 1916, 63, 267. 

Brown—Vertical Copying and Enlarging Apparatus. British Patent 133,- 
_143/1918; Brit. J. Phot., 1920, 67, 39, 259. 


630 PHOTOGRAPHY 


Cuartes—A Method for Exact and Rapid Copying to Scale. Brit. J. Phot., — 


1919, 66, 736. 


CuarLtEs—Determination of Exposures in Copying re Artificial Light. Brit. J. 


Phot., 1922, 69, 700. 

Cierc—Contact Reproductions by Reflected Light (Ullman’s Process). Brit. J. 
Phot., 1921, 68, 65, 645. 

FERRARS—Vertical Stand for Reproduction Work. Camera Craft, 1923, p. 384. 


Gear—Copying Line Subjects. Phot. J., 1916, 56, 177; Brit. J. Phot., 1916, 63, ; 


381. 
HANsEN—Vertical Copying and Enlarging Apparatus. British Patent 135,- 
484/1918; Brit. J. Phot., 1921, 68, 52. 


HeypecKer—Reproduction of Documents by Contact using Reflected Light. — 


(Revival of Playertype.) Brit. J. Phot., 1923, 70, 445. 
MarrraGE—Copying Half-Tone Illustrations. Brit. J. Phot., 1916, 63, 162, 
MULLER AND GANZ—Vertical Copying and Enlarging Apparatus. British Patent 

123,531/1920; Brit. J. Phot., 1920, 67, 304. 


Pascautt—A._ Vertical, Self-Focussing Copying Apparatus using Artificial =a 


Light. British Patent 150,912/1919; Brit. J. Phot., 1920, 67, 633. 
PowEr—Copying to Same Size. Brit. J. Phot., 1916, 63, 439. 


Pratr—Backgrounds for Small Objects Photographed in the Studio. Camera 


Craft, 1923, p. 3. 


RosE—Vertical Arrangement for Copying Small Objects. Brit. J. Phot. 1919, 


66, 338. 

St1LEs—Copying. Amer. Phot., 1922, 16, 634. 

Watit—The Playertype Process. Amer. Phot., 1923, 17, 686. 

Westcotr—Test Object for Sharp Focussing. Amat. Phot., 1921, p. 106. 

WINKLER—Reproductions by Reflected Light. French Patent 556, — 
Dy LoP Seo st ia a 

Miner Mine Focussing by the Parallax Method. Brit. J. Phot., 1917 64, 
322. 

Copying. (Decennia Practica.) Brit. J. Phot., 1916, 63, 7. 


ie 


eee ee ees ee 


SUBJECT INDEX 


Aberration, chromatic, 85 
comatic, 92, 93 
spherical, 89, 90, QI 
zonal, QI 
Absorption densitometers, 231 
Absorption, loss of light in lenses by, 
79 
Accumulator lighting for projection 
printing, 423 
Acetone, 321 
Acetylene, for projection printing, 422 
Achromatic, 89 
lenses, single, 105, 106 
Acid hypo fixing baths, 348 
: extra hardening baths, 350 
troubles with, 350 
Actinometers, 262 
correction for special 
when using, 264 
for carbon printing, 497 
Adapting paper to negative, 396 
Additive methods of trichromatic 
photography, 570 
Adon, 146 
large 142 
Adurol, 2098 
Advertising, lantern slides, 453 
Agfa color plate, 584 
Aldis lenses, 135 
Alkali, acetone as a substitute for, in 
development, 321 
carbonates in development, 319 
caustic, in development, 321 
function in development, 271 
proportion of, to developing agent 
320 
Amidol, 299 
for bromide paper, 399 
preservatives of, 300 
Ammonia, ripening of emulsions with, 
30, 158 
-Ammonium chloride, influence on ra- 
pidity of fixation, 341 


subjects 


Angle of view, 68 
Angstrom unit, 172 
Aperture, effective, 78 
inconstancy of, 79 
relative, variation with subject, 82 
Aplanat, 109 
Apochromat, 89 
Aristostigmat, 120 
Artificial latent images, 201, 202 
light, for copying, 553 
for projection printing, 419 
Astigmatism, 99 
curves, IOI 
Atmosphere, effect on time of expo- 
sure, 254 
Autochrome plate, 577 
after treatment of, 582, 583 
development of, 580 
exposure of, 570 
filters for, 578 
reversal and _ redevelopment 
of, 582 
Aviar, 134 


Back, swing, 43 


- Baynard’s work in photography, 18 


Bichromated colloid printing processes, 
history of, 32 
chemistry of, 492 
Binding lantern slides, 453 
Bis-telar, I41 


Black and white subjects, copying, 
561 
Bleaching, of bromide prints for 


bromoil, 536 
and tanning baths for bromoil, 
separate, 537 
chemical theory of brcmoil, 538 
in sulphide toning, 467 
Blue printing, 487 
Bromide, advantage of excess in emul- 
sions, 155 
density depression with, 285 


631 


632 
effect of soluble, on characteristic 
curve, 285 
effect on the development of oe 
287 
effect on velocity constant and 
gamma infinity, 286 
reaction in development, 271 
Bromide papers, amidol developer for, 
399 
fixing and washing of, 403 
M-Q developer for, 390 
safelight for, 400 
sensitometric characteristics 
of, 390 
Bromoil, the bromide print for, 534 
bleaching, 536, 538 
pigmenting, 539, 542 
producing relief for, 530 
transfer, 543 


CoM." S., 234 
Calotype process, 20 
Camera, box, 35 
copying, 554 
enlarging, 415 
hand, 37 
' miniature, 35 
professional, 39 
reflex, 40 
trichromatic, 571 
Camera obscura, history of, 1 
with lens, 4 
Carbon process, actinometers for, 497 
continuing action of light in, 
499 
development of prints, 499 
exposure in, 407 
sensitizing tissues, 495 
tissues for, 404 
transfer in, double and single, 
495, 500 
transferring to rough papers. 
502 
Carbro process, 503 
bromide print for, 504 
development in, 507 
multiple printing by the, 509 


PHOTOGRAPHY 


sensitizing of tissues in the, 
505, 506 
transfer in the, 507 
Catalytic theory of persulphate reduc- 
tion, 377 
Celluloid as a glazing material, 410 
Celor, 118 ; 
Central speed method, Watkins’, 240 
Characteristic curve, method of. ob- 
taining, 236 
significance of, 237 
Chloranol, 315 
Chromatic aberration, 85 
correction of, 87 
over-correction of, 86 
under-correction of, 86. 
Chemical transfer, Zaepernick’s 
method of, 548 
Chromium intensifier, 383 
for prints, 408 
Clouds in enlargements, 440 
Collinear, 116 


Colloid, bichromated, processes, 480, 


402 
Colloid .silver theory of latent image, 
222 
Collodio-chloride paper, 
31 
Collodion process, Archer’s, 23 
inconveniences of, 23 
modifications of, 25 
Collodion emulsion, 26 
Color-contrasts, photographing, 105 
Color photography, dye bleach proc- 
esses, 568 
light interference processes 
569 
screen plate processes, 575 
trichromatic, 570 
plates, Agfa, 584 
Autochrome, 577 
Duplex, 584 


history of, — 


Condensing lenses, in projection, 425 ic i 


with diffusing media, 428 — 
Conjugate focal distances, 60 | 
in projection printing, — 

427, 436 : 


SUBJECT INDEX 


Constant density ratios, 245 
Contrast, alteration of, with develop- 
ment papers, 406 
alteration of, in the platinotype 
process, 482 
_ gamma as a measure of,. 248 
Cooke triplet anastigmat, 132, 134 
Copper intensification, 385 
Copying, artificial light for, 553 
black and white subjects, 561 
cameras for, 554 
exposure in; 83, 558, 560 
focussing in, 555 
illumination in, 552 
monochromatic subjects, 563 
objectives for, 554 
to scale, 556 
Curvature of field, 94 
Cyanine, 182 


Dagor, 114 
alternate construction of, I15 
Dalmac, 131 
Daguerre, life and work, 12 
Daguerreotype process, 15 
Dallon, 143 
Darkroom, arrangement of, 46 
illumination of, 50 
sinks, 48 
size of, 45 
ventilation, 46 
water supply, 47 
Defatting oil prints, 542 
Defects in negatives, the why of, 359 
Density, Beer’s law, 232 
constant, ratios, 245 
definition of, 232 
dependence on method of meas- 
urement, 231 
growth with exposure, 235 
ratios and opacity ratios, the dif- 
- ference, 246 
Densitometers, H. and D., 230 
absorption and polarization, 231 


Depth of focus, factors controlling, 73 © 


Plasmat and enhanced, 75 
theory of, 7I 


42 


633 


Desensitizing, agents, 322 
of autochromes, 581 
practice of 323 
value of, 322 
Developing agents, classification of, 


280 

reduction potential of, 287, 
289 

relative reducing energy of, 
287 


slow and rapid, 206 
source and manner of deriva- 
tion, 290 
Development, and the reproduttion of 
contrast, 245 
and the perfect negative, 247 
and the structure of gelatin, 267 
as a reversible reaction, 270 
effect of temperature on, 278 
factorial, 272, 325 
function of the alkali in, 271 
induction period of, 271 
invasion phase of, 266 
physical, 208 
precipitation phase of, 269 
reduction phase of, 268 
the latent image in, 260 
thermo, 320 
time of, for given gamma, 275 
time of, for various temperatures, 
284 
velocity of, 272, 273 
Diaphragm, Le Clerc focusing, 556 
systems of notation, 77 
variation in value of, with distance 
of subject, 82 
Dichloric fog, 364 
Dispersion of light, 61, 88 
Dispersoid theory of persulphate re- 
duction, 377 
Distances, conjugate focal, 69 
extra focal, 71 
Distortion, 95 
Dogmar, II9 
Drying, cabinet for, 57 
D-O-P prints, 405 
gum bichromate prints, 515 


634 
lantern slides, 452 
oil, bromoil and transfers, 532 
Duplex method of color photography, 
584 
Duplicating screen plate processes, 
584 
Duratol, 315 
Dye sensitizing, development of, 174 
known facts of, 175 
theory of, 177 
Dyes, local intensification with, 389 
color sensitizing, 178, 179, 180, 
181 | 
Dynar, ‘137 


Edinol, 301 
Effective aperture, 78 
Electron theory of the latent image, 
220 
Emulsion, appearance under micro- 
scope, 163 
advantages of excess halide in, 
155 
centrifugal separation of, 162 
classes of, 149 
digestion of, 157, 159 
emulsification, 155 
fog in, 159 
gelatin in, 151, 152 
grain sensitivity of, 164 
grain-size distribution and prop- 
erties of, 168 
history of gelatine, 26, 28, 29, 30 
iodides in, 157 
molecular states of silver halide 
in, 153 
Eosin, 178 
Ernostar, 138 
Erythrosin, 178 
Exhausting the fixing bath, 344 
Exposing, autochrome plates, 579 
carbon tissues, 497 
developing papers, 396 
for enlarged negatives, 442 
for speed determination, 233 
gum-bichromate prints, 516 
lantern slides, 447 
oil papers, 528 
platinotype papers, 481 


PHOTOGRAPHY 


Exposure, quantum theory of, 165 
Exposure, growth of density with, 235 
Exposure, in negative making, 254 
actinometer, use of, in determin- 
ing, 262 
atmosphere, effect of, on, 254 
methods of determining, 261 


speed of plate, effect of variations 


in, 261 


subject, influence of, 256, 2x7, 258, 


259 
variation 
month and hour, 255 
__ visual exposure meters, 264 
Extra focal distances, 71 
hardening baths, 350 


F system, 77 
Factorial development, accuracy of, 
327 
basis of, 272, 325 
developing factors for, 326 
of bromide papers, 401 
of lantern slides, 451 
Farmer’s reducer, 372 
Filters, autochrome, 578 
contrast, 189 
effect on lens definition, 564 
graduated, I9I 
orthochromatic, 190 


theory of compensating and selec- 


tion, 188 
Fixing, action of sodium thiosulphate 
in, 338 
ammonium chloride, 
rapidity of, 341. 
influence of concentration of hypo 
on rapidity of, 340 


influence of temperature on rate 


of, 340 
mechanism of, 339 


necessity for complete, in sulphur : 


toning, 460 
of P-O-P, 414 
of lantern slides, 452 
of prints, 346, 403 
physical development after, 208 
velocity constant of, 339 


in light intensity by 


effect on i 


, \s 3 " - ~ . 
Se ee ee a ee a en 


eS eee a ee ee 


SUBJECT INDEX 635 


Fixing bath, acid, 348 
acid fixing and hardening, 349 
troubles with, 350 
exhaustion of, 344 
extra hardening, 350 
plain, 347 
Flare and flare spot, 102 
Focal length, 65 
and size of image, 66 
Focal distances, conjugate, 69 
in projection printing, 427 
Focus, 65 
Focusing, diaphragm for, 556 
in projection printing, 437 
parallax method, 556 
use of swing back in, 43 
Fog, chemical, 362 
dichloric, 364 
emulsion, 159 
general light, 361 
local, 360 


Gamma, calculation of, 250 
definition of, 248 
mathematical expression for, 249 
relation to the characteristic curve, 
249 
time of development for given, 
calculating, 275, 277 
Gamma infinity, 251 
bromide, effect of soluble on, 286 
determination of, 252 
emulsion, effect of, 251 
Gauss theory, image formation ac- 
cording to, 63 
Gaussian objectives, 119, 120 
Gelatine, physical properties of, 151 
photographic properties, 152 
gelatine-X, 152 
Gelatino-citro-chloride paper, 31 
Glass, introduction of, as negative ma- 
terial, 21 
Glycin, 303 
time development formula, 334 


Green tones, with iron, 476 


with vanadium, 476 
Group relations, effect on developing 
property, 294 


Gum-bichromate process, coating 
papers, 514 
development, 516 
drying sensitized paper, 515 
exposure, 516 
formulas for coating, 512 
materials for working the, 512 
registration in multiple print- 
ing, 517 
the negative for, 512 
Gum-bromide, 519 
Gum-platinum, 519 


Halogen, evidences for liberation of, 
on exposure, 215 
Hard printing papers, 304 
Heliar, 136 
Homocol, 181 
Hydrochinon, 304 
contrast formula, 305 
reactions in development, 269 
Hypo, action on silver halides, 338 
eliminators, 357 
tests for presence of, 356 
toning with acid, 464 
Hypo-alum, toning with, 461 
accelerated methods of ton- 
ing, 462 
controlled methods of toning, 
462 


Illuminants for projection printing, 
419 
Illumination of the darkroom, 50 
Illumination, unequal, 96 
Image formation according to the 
Gauss theory, 63 
Image, intensity of optical, 75 
transference of, 209 
Inconstancy of aperture, 79 
Indirect sulphide toning, 467 
and redevelopment, 470 
Indoxyl development, 210 
Induction period in development, 271 
Inertia, as a measure of speed, 238 
variation of, 239 
Inspection, development by, 324 
Instantaneous toning of P-O-P, 412 
Intensification, how secured, 379 


636 
local with dyes, 388, 389 


of lantern slides, 456 
of prints, 407 
sensitometry of, 386 
Intensifying processes, 
387 ) 
chromium, 383 
copper, 385 
lead, 385 
mercuric-iodide, 381 
mercury, 379, 380 
Monckhoven’s method, 381 
silver, 382 
sulphide, 386 
uranium, 384 
Intermittency in exposure, effect of, 
229 
Invasion phase of development, 266 
Iron printing processes, 486 
toning processes, 475 
Isostigmar, 125 


classification, 


Kallitype process, 486 


Landscape ‘photography, orthochro- 


matic methods in, 191 


Lantern slide, making advertising, 453 


binding, 453 

by reduction, 447 

developers and development, 
449, 451 

exposing, 447 

fixing, washing and drying, 
452 

masking, 452 

negative for, 445 

physical development of, 455 

plates for, 445 

printing frame for contact 
printing of, 446 

reduction of, 456 

spotting, 452 

thiocarbamide, development 
with, for warm tones, 455 

toning of, 456 

warm tones by development 
on, 454 


PHOTOGRAPHY 


Latent image, artificial, 201 
formation of, at low tem- 
peratures, 220 
function of, in development, 
269 
halogenizing agents, action on, 
211 
indoxyl development of, 210 
oxidizing agents, action on, 
2iI 
photoregression, 206 
photosalts, 209 . 
reversal of the, 204, 205 
silver solvents, action on, 207 
transference of, 200 
Latent image theories, 211 
colloid-silver theory, 222 
electron theory, 220 
metallic silver theory, 217 
molecular strain theory, 218 
orientation hypothesis, 223 
‘oxy-halide theory, 212 
sub-halide theory, 214 
Latitude of sensitive materials, 243 
Lead, intensification with, 385 
Lenses for copying, 554 
Lenses for projection printing, 429 
Lenses, image formation by, 62 
Lenses, loss of light in, by seesicten: 
and reflection, 79 
Lenses, speed of, 77 
Light, and color, 172 
continuing action of, 499 
dispersion of, 61 
refraction of, 50 
Light filter, autochrome, 578 
compensating, 190 
contrast, 189 
graduated, 191 
theory of, 188 
Light-interference processes of color — 
photography, 560 ey 


Local reduction and intensification, 388 hast 


bench for, 389 
Luminosity, visual and photochemical, 
173 ioe 


Magnar, 142 


SUBJECT INDEX 


Maximum black, of printing papers, 
392 

Mercuric-iodide intensifier, 381 

Mercury intensification, 379 

Mercury-sulphide toning, 471 

Metallic silver theory of the latent 
image, 217 

Metol, 306 

Metoquinone, 308 

Molecular strain theory of the latent 
image, 218 

Monckhoven’s intensifier, 381 

Monochrome subjects, copying, 563 

Monomet, 308 


Negative, H. & D. definition of per- 
fect, 241 
for gum-bichromate, 512 
for lantern slides, 445 
for projection printing, 433 
' Neol, 308 
Neostigmar, 126 


Objective, achromatic, single, 105, 106 
Objective, aplanatic, 109 
Objective, cemented symmetrical an- 
astigmatic, 114 
Collinear, 116 
Dagor, 114 
Holostigmat, 115— 
Orthostigmat, 116 
Protar, 116 
Turner-Reich, 117 
Objective, Gaussian anastigmatic, 118 
Aristostigmat, 120 
Homocentric, 121 
Omnar, 121 
Opic, 121 
Planar, 119 
Objective, meniscus, 104 
Objective, Petzval portrait, 110 
: modifications of, 112 
Objective, semi-achromatic, 107 
Objective, symmetrical air space. an- 
astigmatic, 118 
Celor, 118 
Dogmar, I19 
Syntor, 118 


637 
Objective, telephoto, 138 
adon, 145 
anastigmatic, 142, 143, 144, 
145 


compound, 139 
early fixed magnification, 141, 
142 
Objective, triple, 108, 109 
Objective, triple anastigmatic, 132 
Aldis, 135 
Aviar, 134 
Cooke, 132 
Dynar, 137 
Ernostar, 138 
Heliar, 136 
Pentac, 137 
Objective, unsymmetrical anastig- 
matic, 127 
Dalmac, 131 
Protar, 127 
Radiar, 131 
Serrac, 130 
Tessar, 120 
Unar, 128 
X-press, 131 
Oil process, 524 
brushes for, 525 
drying, 532 
exposing, 528, 531 
papers for, 525 
pigmenting in, 520 
pigments for, 526 
sensitizing, 527 
Organic developing agents, classifica- 
tion of, 289 
influence of. group rela- 
tions on, 204 
slow and rapid, 206 
source of, 290 
Orthochrom T, 179 
Orthoscopic lens, 141 
Ortol, 309 
Oxy-halide theory of the latent image, 
212 
Ozobrome, 491 
Ozotype, 490 


Panchromatic sensitizing, 178 


638 


Paper, early negative processes, 20 
Paper, printing, bromide, 32, 390 
bromoil, 33, 533 : 
developing, 32, 390 
iron, 486, 487 
oil, 525 
platinum, 480 
P-O-P, 411 
Satista, 484 
silver-platinum, 484 
Papers, printing, adapting to negative, 
304 
sensitometric 
390 
Parallax focusing method, 556 
Paramidophenol, 310 . 
Pentac, 137 
Permanganate, as hypo eliminator, 357 
Permanganate, reduction, 374, 408 
Permanganate, test for hypo, 356 
Persulphate reduction, catalytic theory 
of, 377 
dispersoid theory of, 377 
practice of, 377 
sensitometric action of, 371 
Peroxide, artificial latent images with, 
202 
Perspective, 67 
Petzval portrait lens, 110 : 
Phenosafranine, 322 
Photochemical action, 


constants of, 


early records 


of, 5 
Photo-electric effect, 220 
Photophysical and _ photochemical 


change, 200 
Photoregression, 206 
Physical development, after fixation, 
208 
of lantern slides, 455 


Pigmenting, of bromoil prints, 539, 


542 
of oil prints, 529 


relation to character of the image, 


540 
Pinachrome, 180 
blue, 180 
violet, 181 
Pinakryptol, 322 


"PHOTO GRAPHY 


Plasmat, 122 rs. 
Plate, agfa, sf 


orthockrons aa ‘¥ 
panchromatic, 178 
process, 561 _— 
wet, 23 


482 +P ace 
Polysulphides, nee wit 
Powder processes, 520 
Precipitation phase of 
269 . ee 
Principal foe 65 2 
Printing, bromide, 390 
bromoil, 533 
carbon, 493. 
-carbré; 503 
gum-bichromate, II . 
iron, 4865 <-..9unee 
lantern slides, 4 Aq 

Oil, S842 oe 

5 platinotype, 479 we 
P- O- P; 411 


419 
eee in, : 
_ daylight for, 
Oe fof oe 


‘tana for a : 


SUBJECT INDEX 


Quantum theory of photographic ex- 
posure, 165 


Radiar, 131 
Rebleaching of sulphide toned prints, 
470 
Redevelopment of bromide print after 
carbro, 507 
Redevelopment, sulphide toning with 
intermediate, 470 
Reducers, Belitzski’s, 373 
Farmer’s, 372 
iodine-cyanide, 374 
mercury-cyanide, 373 
permanganate, 374, 408 
persulphate, 377, 408 
proportional, 371, 375 
subtractive, 371 
superproportional, 371, 376 
Reduction, chemical, in development 
with hydrochinon, 269 
Reduction, lantern slides by, 447 
Reduction, local, 388 
of prints, 407 
Reduction potential 
agents, 287 
Refraction of light, 59 
Reflection, loss of light in lenses from, 
79 
Relative aperture, variation with sub- 
ject, 82 
Relative exposures in copying and en- 
larging, 82, 439 
Rendering power of printing papers, 
392 | 
Resinopigmentype, 521 
Restrained development, warm tones 
on slides by, 454 
Reversal, of autochroms, 582 
of latent image by chemical re- 
agents, 205 


of developing 


of latent image by continued ex- 


posure, 204 
Rough papers, transferring carbon 
prints to, 502 


Safelight, efficiency of, 53 
for developing papers, 400 


639 


Satista paper, 484 
Schellenwert method of speed deter- 
mination, 226 
Screen plates, 575 
Secondary spectrum, 89 
Sensitivity, centers, 224 
nature of the substance, 167 
of the silver halide grain, 164 
Sensitizing blue print paper, 487 
carbon tissue, 405 
carbro tissue, 506 
oil papers, 527 
Sensitizing, dye, history of, 174 
known facts of, 175 
theories of, 177 
Sensitizing substance in gelatine, 152 
Sensitometry, central speed method, 
240 
characteristic curve, 236 
constant density ratios, 245 
correct reproduction, 242 
definition of, 226 
densitometers, 229 
density-exposure relation, 242 
developers and development in, 
233 
development and contrast, 247 
development and reproduction of 
contrast, 245 
gamma, 248 
H. & D. system, 226 
inertia, 238, 239 
latitude, 243 
perfect negative, 241 
Schellenwert method of, 226 
sensitometers, 228, 229 
standard light sources, 227 
Sensitometry of intensification, 386 
of printing papers, 390 
Serrac, 130 
Silver, action of solvents of, on latent 
image, 207 
Silver halide, action of hypo on, 338 
sensitivity of grain of, 164 
Silver intensifiers, 382 
Silver-platinum papers, 484 
Silver printing processes, history of, 
30 


640 


Silver stains on negatives, 367 
Silver, sub-halides of, 215 
Silver salts, light sensitiveness of, 153 
Sinks, 48 
Spectrum, 172 
Speed of lenses, 77 
Spherical aberration, 89 
Spots on negatives, opaque, 369 
transparent, 368 
yellow, 370 
Stains on negatives, developer, 365 
miscellaneous, 367 
silver, 367 
Starch-iodide test for hypo, 357 
Stigmatic, 126 
Sub-halide, theory of the latent image, 
214, 216 
Sub-halides, evidence for the exist- 
ence of, 215 
Subtractive printing processes, 575 
reducers, 371 
Sulphite, control of developer stain 
with, 319 
forms of, 317 
stock solutions of, 318 
theory of action in development, 
270 


Sulphur toning, direct, 461, 462, 463, 


464, 465, 466 
indirect, 467 
indirect with redevelopment, 
170 OO 
influence of development, 459 
influence of emulsion, 460 
mercury-sulphide, 471 
necessity for complete fixing, 
460 
rebleaching of toned prints, 
470 
Superproportional reducers, 371 
Swing back, 43 


Table, correspondence of systems of 
plate marking, 260 
light sensitiveness of silver salts, 
153 
of factors required in determine 
tion of k, 279 


PHOTOGRAPHY 


variation in light intensity by — 
month and hour, 255 
Telecentric, 142 
Tele-Dynar, 145 
Telegor, 144 
Teleobjective, anastigmatic, 142, 143, 
144, 145 
compound, 139 
early fixed-magnification, 141, 142 
principle of, 138 
Teleros, 145 
Teletessar, 144 
Telic, 143 
Temperature, calculation of time of 
development for given, 284 
sensitiveness of plate at low, 220 
Temperature coefficient, calculation of, 
281 
‘ factors controlling, 281 
mathematical expression for, 
280 
of developing agents, 281 
Tessar, 129 
constructions based on, 130 
spherical aberration of, 92 
Thermo development, developers and — 
tables for, 332 
efficiency of, 335 
principles of, 329 
with glycin, 334 
Thiocarbamide, development with ad- 
dition of, 455 


Time of development at various tem- 


peratures, calculating, 284 
for given gamma, finding, 275 
Tissues, carbon, 494 
Toning, accelerated hypo-alum, 462 — 
hypo-alum, 461, 462 
indirect sulphide, 467 
indirect sulphide with redevelop- 
ment, 470 
iron, 475 
liver of sulphur, 464 
mercury-sulphide, 471 
nitro-sulphide, 466 
of lantern slides, 456 
of P-O-P, 411 * 
polysulphide, 464 


SUBJECT INDEX 641 


uranium, 473 
vanadium, 476 
Total scale of printing papers, 392 
Transfer, carbon, 495, 500, 502 
carbro, 507 
multiple bromoil, 543, 549 
papers, 495, 545 
presses, 546 
Zaepernick’s chemical transfer 
method, 548 
Transparency, sensitometric definition 
of, 231 
Trichromatic color photography, 570 
negative making, 571 
printing processes, 573 
Triplet, anastigmatic objectives, 132, 
134 
‘Dallmeyer’s, 109 
Sutton’s, 109 
Trox film washer, 354 


U. S. system, 77 


Unar, 128 


Unequal illumination of image, 96 
Unofocal, 123 


Uranium intensification, 384 
Uranium toning, 473 


Vanadium, toning with, 476 
Varnishing of autochroms, 583 
Velocity constant, 274, 275 
Velocity of development, 272 
formula for, 274 
method of determining, 273 
Velocity of fixation, 339 
View, angle of, 68 


Washers, centrifugal, for prints, 405 
Neblette’s, for cut film, 354 
rotary for prints, 405 
Trox for roll film, 354 
Windoe’s, for plates, 353 

Washing, efficiency of devices, 352 
mechanism of, 351 
of prints, 355, 405 

Watkins factor, for common develop- 

ing agents, 326 
controlling influences, 326 
theory of, 272 


INDEX OF AUTHORS 


Abegg, 208, 217 

Abney, 31, 155, 157, 173, 205, 261, 338 
Aldis, 135 

Allen, 222 

Anderson, 294, 500, 514 

Aristotle, 1 


Bacon, 2 
Baekeland, 32 
Baynard, 18 

Beck, 125 

Beechey, 26 
Belitski, 373 
Bennett, 20, 155, 386, 471 
Blake-Smith, 466 
Bolton & Sayce, 26 
Booth, 143 

Bow, 98 

Brown, 97, 98 
Bullock, 465, 468 
Butler 572 


Callier, 410, 433 
Charles, 556, 550 
Cheshire, 80 
Clark, 167 
Collins, 4390 
Cros, 560, 572 


Daguerre, 12 

Dallmeyer, 106, 107, 109, I12, 139, 
145 

Da Vinci, 2 

Debenham, 83 

Dewar, 220 

Dillaye, 581 

Draper, 17, 568 

Drinkwater, 462 

Du Hauron, 573, 574, 575, 576 

Duvivier, 533 


Eder, 104, 155, 158, 160, 175, 176, 200, 
208, 373 


Eder & Toth, 472 


Farmer, 372, 491, 492 
Ferguson, 278, 472 


Fox-Talbot, 18, 19, 20, 489 


Gard, 428 
Gleichen, 104 
Glover, 535 
Goddard, 106 
Graf, 124 
Greenall, 467, 471 
Grubb, 106 
Guttmann, 539 


H & D, 277, 433 
Harting, 104, 136 


Herschel, 1, 21, 32, 487, 568 
Hickman & Speveee 355 


Hodgson, 165 
Hoegh, 115, 118 
Homolka, 210, 214 
Houdaille, 278 
Hunt, 32, 568 


Inston, 484 


Ives, 560, 571, 572, 574, 584 


Jobling & Salt, 79 
Johnson, 29, 30 


Jones, Chapman, 221, 433 


Kaempfer, 116 
Kellner, 91, 92, 100 
Kennett, 29 

Kepler, 4 

King, 438 
Kollmorgen, 120 
Konig, 175 

Kropf, 466 


Lambert, 438 
Lea, Carey, 200, 214 
Le Clerc, 386, 556 


642 


I ee ee 2 ee 


INDEX OF AUTHORS 


Lee, 121, 143 

ee ibtay 3 

Liesegang, 560 

Lippmann, 596 

Lockett, 327, 438 

Lumiére & Seyewetz, 208, 204, 342, 
344, 345, 365, 376, 460, 465, 538, 576 

Luppo-Cramer, 167, 176, 209, 222, 377 

Luther, 160, 240 


Maddox, 26 

_ Manly, 33 

Mansfield, 29 

Marion, 490 

Martin, 121 

Maxwell, Clerk, 571 

Mayer, 534, 536, 544, 548 

McDonough, 576 

Mees, 169, 192 

Mees and Gutekunst, 175 

Mees and Sheppard, 227, 252, 275, 278, 
351 

Meldola, 174, 338 

Mercator, 208 

Monckhoven, 158 

Mortimer, 483 


Namias, 201, 374, 472, 476, 521, 534, 
542 

Neitz & Huse, 371,. 375, 386 

Neuhaus, 506 

Nicol, 486 

Niepce, 8, 11 

Norris, 25 


Oswald, 217 
Owen, 517 


Partington, 540 

Petzval, 110 

Piper, 33, 79, 340, 342, 492 
Poitevin, 33, 480, 401, 568 
Ponton, 32, 480 

Pope, 175, 182 

“Yorta, 3 

Pouncy, 32, 489 

Prett, 547 

Punnett, 466 


643 


Rawlings, 33, 492 

Reinders, 214 

Renwick, 167, 209, 210, 222 

Rohr, 75, 104 

Ross, I10 

Rudolph, 116, 119, 122, 127, 128, 120 
Russell, 202 


Scheele, 7, 214 
Scheffer, 377 

Schuler, 377 

Schulze, 5 
Scott-Archer, 21 
Sedlaczek, 384, 474 
Shaw, 466 

Sheppard, 168, 223 
Sheppard & Wightman, 167, 160 
Simpson, 31, 568 
Smith, 569 

Snodgrass, 399 

Stas, 153 

Steinheil, 109, 116, 123 
Stenger, 377 

Stokes, 285 

Svedberg, 165 

Swan, 33, 490 

Symes, 543 


Taylor, 132 
Thomson, 485 
Triepel, 466 
Trivelli, 214 


Underberg, 465 


Vallot, 569 

Venn, 537, 538 
Vogel, 174 
Voigtlander, 110, 112 


Wall, 33, 477, 492, 520 
Warmisham, 134 

Waterhouse, 175 

Watkins, 240, 278, 282, 325, 320 
Wedgwood, 8 

Wellington, 382 


644 


Willis, 32 
Windoes, 353 
Worel, 569 


Young, 570 


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